feat(gateway/slack): support channel_skill_bindings

Extends the existing channel_skill_bindings mechanism (previously
Discord-only) to Slack, so a channel or DM can auto-load one or more
skills at session start without relying on the model's skill selector
for every short reply.

Motivation: Mats's German flashcards DM pushes a cron-driven card
5x/day; he responds with one-word guesses like 'work'. Previously each
reply required the main agent to decide whether to load german-flashcards
(full opus turn just to pick a skill). With the binding configured per
Slack channel, the skill is injected at session start and grading runs
directly.

Changes:
- Extract resolve_channel_skills() from DiscordAdapter._resolve_channel_skills
  into gateway.platforms.base (now shared across adapters).
- DiscordAdapter._resolve_channel_skills delegates to the shared helper
  (behavior preserved — existing test suite still passes unchanged).
- SlackAdapter: resolve channel_skill_bindings on each message and attach
  auto_skill to MessageEvent. gateway/run.py already handles auto-skill
  injection on new sessions; this just wires Slack through it.
- gateway/config.py: accept channel_skill_bindings in slack: block of
  config.yaml (was Discord-only).
- Tests: new tests/gateway/test_slack_channel_skills.py with 11 cases
  covering DM/thread/parent resolution, single-vs-list skills, dedup,
  malformed entries. Discord suite unchanged.
- Docs: add 'Per-Channel Skill Bindings' section to Slack user guide.

Config example:
  slack:
    channel_skill_bindings:
      - id: "D0ATH9TQ0G6"
        skills: ["german-flashcards"]

gateway/config.py

@@ -598,7 +598,7 @@ def load_gateway_config() -> GatewayConfig:
                     bridged["group_policy"] = platform_cfg["group_policy"]
                 if "group_allow_from" in platform_cfg:
                     bridged["group_allow_from"] = platform_cfg["group_allow_from"]
-                if plat == Platform.DISCORD and "channel_skill_bindings" in platform_cfg:
+                if plat in (Platform.DISCORD, Platform.SLACK) and "channel_skill_bindings" in platform_cfg:
                     bridged["channel_skill_bindings"] = platform_cfg["channel_skill_bindings"]
                 if "channel_prompts" in platform_cfg:
                     channel_prompts = platform_cfg["channel_prompts"]

gateway/platforms/base.py

@@ -990,6 +990,61 @@ def resolve_channel_prompt(
     return None
 
 
+def resolve_channel_skills(
+    config_extra: dict,
+    channel_id: str,
+    parent_id: str | None = None,
+) -> list[str] | None:
+    """Resolve auto-loaded skill(s) for a channel/thread from platform config.
+
+    Looks up ``channel_skill_bindings`` in the adapter's ``config.extra`` dict.
+
+    Config format::
+
+        channel_skill_bindings:
+          - id: "C0123"          # Slack channel ID or Discord channel/forum ID
+            skills: ["skill-a", "skill-b"]
+          - id: "D0ABCDE"
+            skill: "solo-skill"  # single string also accepted
+
+    Prefers an exact match on *channel_id*; falls back to *parent_id*
+    (useful for forum threads / Slack threads inheriting the parent channel's
+    binding).
+
+    Returns a deduplicated list of skill names (order preserved), or None if
+    no match is found.
+    """
+    bindings = config_extra.get("channel_skill_bindings") or []
+    if not isinstance(bindings, list) or not bindings:
+        return None
+    ids_to_check: set[str] = set()
+    if channel_id:
+        ids_to_check.add(str(channel_id))
+    if parent_id:
+        ids_to_check.add(str(parent_id))
+    if not ids_to_check:
+        return None
+    for entry in bindings:
+        if not isinstance(entry, dict):
+            continue
+        entry_id = str(entry.get("id", ""))
+        if entry_id in ids_to_check:
+            skills = entry.get("skills") or entry.get("skill")
+            if isinstance(skills, str):
+                s = skills.strip()
+                return [s] if s else None
+            if isinstance(skills, list) and skills:
+                seen: list[str] = []
+                for name in skills:
+                    if not isinstance(name, str):
+                        continue
+                    nm = name.strip()
+                    if nm and nm not in seen:
+                        seen.append(nm)
+                return seen or None
+    return None
+
+
 class BasePlatformAdapter(ABC):
     """
     Base class for platform adapters.

gateway/platforms/discord.py

@@ -2679,21 +2679,8 @@ class DiscordAdapter(BasePlatformAdapter):
                 skills: ["skill-a", "skill-b"]
         Also checks parent_id so forum threads inherit the forum's bindings.
         """
-        bindings = self.config.extra.get("channel_skill_bindings", [])
-        if not bindings:
-            return None
-        ids_to_check = {channel_id}
-        if parent_id:
-            ids_to_check.add(parent_id)
-        for entry in bindings:
-            entry_id = str(entry.get("id", ""))
-            if entry_id in ids_to_check:
-                skills = entry.get("skills") or entry.get("skill")
-                if isinstance(skills, str):
-                    return [skills]
-                if isinstance(skills, list) and skills:
-                    return list(dict.fromkeys(skills))  # dedup, preserve order
-        return None
+        from gateway.platforms.base import resolve_channel_skills
+        return resolve_channel_skills(self.config.extra, channel_id, parent_id)
 
     def _resolve_channel_prompt(self, channel_id: str, parent_id: str | None = None) -> str | None:
         """Resolve a Discord per-channel prompt, preferring the exact channel over its parent."""

gateway/platforms/slack.py

@@ -1759,10 +1759,13 @@ class SlackAdapter(BasePlatformAdapter):
         )
 
         # Per-channel ephemeral prompt
-        from gateway.platforms.base import resolve_channel_prompt
+        from gateway.platforms.base import resolve_channel_prompt, resolve_channel_skills
         _channel_prompt = resolve_channel_prompt(
             self.config.extra, channel_id, None,
         )
+        _auto_skill = resolve_channel_skills(
+            self.config.extra, channel_id, None,
+        )
 
         # Extract reply context if this message is a thread reply.
         # Mirrors the Telegram/Discord implementations so that gateway.run
@@ -1791,6 +1794,7 @@ class SlackAdapter(BasePlatformAdapter):
             reply_to_message_id=thread_ts if thread_ts != ts else None,
             channel_prompt=_channel_prompt,
             reply_to_text=reply_to_text,
+            auto_skill=_auto_skill,
         )
 
         # Only react when bot is directly addressed (DM or @mention).