fix(Slack): resolve Slack channels by raw ID and enumerate joined channels

send_message(target='slack:<channel_id>') failed with "Could not
resolve" because _parse_target_ref had no Slack branch — Slack's
uppercase alphanumeric IDs fell through to channel-name resolution,
which only matched by name. As a fallback, the agent would retry with
bare target='slack' and post to the home channel instead.

Three fixes:

- _parse_target_ref recognizes Slack IDs (C/G/D/U/W prefix) as
  explicit targets so the name-resolver is bypassed entirely.
- resolve_channel_name tries a case-sensitive raw-ID match before
  the existing name match, so any platform's IDs resolve cleanly.
- _build_slack now actually calls users.conversations against each
  workspace's AsyncWebClient (paginated), instead of only returning
  session-history entries. This populates the directory with public
  and private channels the bot has joined, so action='list' shows
  them and they can also be addressed by name. Errors from one
  workspace don't block others.

build_channel_directory becomes async (Slack web calls require it).
The two async-context callers in gateway/run.py are awaited; the
cron ticker thread call bridges via asyncio.run_coroutine_threadsafe.

Slack bot needs channels:read and groups:read scopes for full
enumeration; missing scopes degrade gracefully per-workspace.

addressing #15927

tests/gateway/test_channel_directory.py

@@ -1,9 +1,11 @@
 """Tests for gateway/channel_directory.py — channel resolution and display."""
 
+import asyncio
 import json
 import os
 from pathlib import Path
-from unittest.mock import patch
+from types import SimpleNamespace
+from unittest.mock import AsyncMock, MagicMock, patch
 
 from gateway.channel_directory import (
     build_channel_directory,
@@ -12,6 +14,7 @@ from gateway.channel_directory import (
     format_directory_for_display,
     load_directory,
     _build_from_sessions,
+    _build_slack,
     DIRECTORY_PATH,
 )
 
@@ -62,7 +65,7 @@ class TestBuildChannelDirectoryWrites:
         monkeypatch.setattr(json, "dump", broken_dump)
 
         with patch("gateway.channel_directory.DIRECTORY_PATH", cache_file):
-            build_channel_directory({})
+            asyncio.run(build_channel_directory({}))
             result = load_directory()
 
         assert result == previous
@@ -142,6 +145,21 @@ class TestResolveChannelName:
         with self._setup(tmp_path, platforms):
             assert resolve_channel_name("telegram", "Coaching Chat / topic 17585") == "-1001:17585"
 
+    def test_id_match_takes_precedence_over_name(self, tmp_path):
+        """A raw channel ID resolves to itself, even when a different
+        channel happens to be named the same string. Case-sensitive: Slack
+        IDs are uppercase and must not be normalized away."""
+        platforms = {
+            "slack": [
+                {"id": "C0B0QV5434G", "name": "engineering", "type": "channel"},
+                {"id": "C99", "name": "c0b0qv5434g", "type": "channel"},
+            ]
+        }
+        with self._setup(tmp_path, platforms):
+            assert resolve_channel_name("slack", "C0B0QV5434G") == "C0B0QV5434G"
+            # Lowercase still falls through to name matching (case-insensitive)
+            assert resolve_channel_name("slack", "c0b0qv5434g") == "C99"
+
     def test_display_label_with_type_suffix_resolves(self, tmp_path):
         platforms = {
             "telegram": [
@@ -332,3 +350,135 @@ class TestLookupChannelType:
         }
         with self._setup(tmp_path, platforms):
             assert lookup_channel_type("discord", "300") is None
+
+
+def _make_slack_adapter(team_clients):
+    """Build a stand-in for SlackAdapter exposing only ``_team_clients``."""
+    return SimpleNamespace(_team_clients=team_clients)
+
+
+def _make_slack_client(pages):
+    """Build an AsyncWebClient mock whose ``users_conversations`` returns pages."""
+    client = MagicMock()
+    client.users_conversations = AsyncMock(side_effect=pages)
+    return client
+
+
+class TestBuildSlack:
+    """_build_slack actually calls users.conversations on each workspace client."""
+
+    def test_no_team_clients_falls_back_to_sessions(self, tmp_path):
+        sessions_path = tmp_path / "sessions" / "sessions.json"
+        sessions_path.parent.mkdir(parents=True)
+        sessions_path.write_text(json.dumps({
+            "s1": {"origin": {"platform": "slack", "chat_id": "D123", "chat_name": "Alice"}},
+        }))
+
+        with patch.dict(os.environ, {"HERMES_HOME": str(tmp_path)}):
+            entries = asyncio.run(_build_slack(_make_slack_adapter({})))
+
+        assert len(entries) == 1
+        assert entries[0]["id"] == "D123"
+
+    def test_lists_channels_from_users_conversations(self, tmp_path):
+        client = _make_slack_client([
+            {
+                "ok": True,
+                "channels": [
+                    {"id": "C0B0QV5434G", "name": "engineering", "is_private": False},
+                    {"id": "G123ABCDEF", "name": "secret-chat", "is_private": True},
+                ],
+                "response_metadata": {},
+            },
+        ])
+        with patch.dict(os.environ, {"HERMES_HOME": str(tmp_path)}):
+            entries = asyncio.run(_build_slack(_make_slack_adapter({"T1": client})))
+
+        ids = {e["id"] for e in entries}
+        assert ids == {"C0B0QV5434G", "G123ABCDEF"}
+        types = {e["id"]: e["type"] for e in entries}
+        assert types["C0B0QV5434G"] == "channel"
+        assert types["G123ABCDEF"] == "private"
+        client.users_conversations.assert_awaited_once()
+
+    def test_paginates_via_response_metadata_cursor(self, tmp_path):
+        client = _make_slack_client([
+            {
+                "ok": True,
+                "channels": [{"id": "C001", "name": "first", "is_private": False}],
+                "response_metadata": {"next_cursor": "cur1"},
+            },
+            {
+                "ok": True,
+                "channels": [{"id": "C002", "name": "second", "is_private": False}],
+                "response_metadata": {"next_cursor": ""},
+            },
+        ])
+        with patch.dict(os.environ, {"HERMES_HOME": str(tmp_path)}):
+            entries = asyncio.run(_build_slack(_make_slack_adapter({"T1": client})))
+
+        assert {e["id"] for e in entries} == {"C001", "C002"}
+        assert client.users_conversations.await_count == 2
+
+    def test_per_workspace_error_does_not_block_others(self, tmp_path):
+        bad = MagicMock()
+        bad.users_conversations = AsyncMock(side_effect=RuntimeError("boom"))
+        good = _make_slack_client([
+            {
+                "ok": True,
+                "channels": [{"id": "C999", "name": "ok-channel", "is_private": False}],
+                "response_metadata": {},
+            },
+        ])
+        with patch.dict(os.environ, {"HERMES_HOME": str(tmp_path)}):
+            entries = asyncio.run(_build_slack(_make_slack_adapter({"BAD": bad, "GOOD": good})))
+
+        assert {e["id"] for e in entries} == {"C999"}
+
+    def test_session_dms_merged_when_not_in_api_results(self, tmp_path):
+        sessions_path = tmp_path / "sessions" / "sessions.json"
+        sessions_path.parent.mkdir(parents=True)
+        sessions_path.write_text(json.dumps({
+            "s1": {"origin": {"platform": "slack", "chat_id": "D456", "chat_name": "Bob"}},
+            "dup": {"origin": {"platform": "slack", "chat_id": "C001", "chat_name": "first"}},
+        }))
+        client = _make_slack_client([
+            {
+                "ok": True,
+                "channels": [{"id": "C001", "name": "first", "is_private": False}],
+                "response_metadata": {},
+            },
+        ])
+        with patch.dict(os.environ, {"HERMES_HOME": str(tmp_path)}):
+            entries = asyncio.run(_build_slack(_make_slack_adapter({"T1": client})))
+
+        ids = {e["id"] for e in entries}
+        assert "C001" in ids and "D456" in ids
+        # Channel ID from API should not be duplicated by the session merge
+        assert sum(1 for e in entries if e["id"] == "C001") == 1
+
+    def test_skips_channels_with_no_id_or_name(self, tmp_path):
+        client = _make_slack_client([
+            {
+                "ok": True,
+                "channels": [
+                    {"id": "C001", "name": "good", "is_private": False},
+                    {"id": "", "name": "no-id"},
+                    {"id": "C002"},  # no name (e.g. IM)
+                ],
+                "response_metadata": {},
+            },
+        ])
+        with patch.dict(os.environ, {"HERMES_HOME": str(tmp_path)}):
+            entries = asyncio.run(_build_slack(_make_slack_adapter({"T1": client})))
+
+        assert {e["id"] for e in entries} == {"C001"}
+
+    def test_response_not_ok_breaks_pagination_for_that_workspace(self, tmp_path):
+        client = _make_slack_client([
+            {"ok": False, "error": "missing_scope"},
+        ])
+        with patch.dict(os.environ, {"HERMES_HOME": str(tmp_path)}):
+            entries = asyncio.run(_build_slack(_make_slack_adapter({"T1": client})))
+
+        assert entries == []

tests/tools/test_send_message_tool.py

@@ -810,6 +810,40 @@ class TestParseTargetRefE164:
         assert _parse_target_ref("matrix", "+15551234567")[2] is False
 
 
+class TestParseTargetRefSlack:
+    """_parse_target_ref recognizes Slack channel/user IDs as explicit."""
+
+    def test_public_channel_id_is_explicit(self):
+        chat_id, thread_id, is_explicit = _parse_target_ref("slack", "C0B0QV5434G")
+        assert chat_id == "C0B0QV5434G"
+        assert thread_id is None
+        assert is_explicit is True
+
+    def test_private_channel_id_is_explicit(self):
+        assert _parse_target_ref("slack", "G123ABCDEF")[2] is True
+
+    def test_dm_id_is_explicit(self):
+        assert _parse_target_ref("slack", "D123ABCDEF")[2] is True
+
+    def test_user_id_is_explicit(self):
+        assert _parse_target_ref("slack", "U123ABCDEF")[2] is True
+        assert _parse_target_ref("slack", "W123ABCDEF")[2] is True
+
+    def test_whitespace_is_stripped(self):
+        chat_id, _, is_explicit = _parse_target_ref("slack", "  C0B0QV5434G  ")
+        assert chat_id == "C0B0QV5434G"
+        assert is_explicit is True
+
+    def test_lowercase_or_short_id_is_not_explicit(self):
+        assert _parse_target_ref("slack", "c0b0qv5434g")[2] is False
+        assert _parse_target_ref("slack", "C123")[2] is False
+        assert _parse_target_ref("slack", "X0B0QV5434G")[2] is False
+
+    def test_slack_id_not_explicit_for_other_platforms(self):
+        assert _parse_target_ref("discord", "C0B0QV5434G")[2] is False
+        assert _parse_target_ref("telegram", "C0B0QV5434G")[2] is False
+
+
 class TestSendDiscordThreadId:
     """_send_discord uses thread_id when provided."""