fix(discord): complete #18741 for /skill autocomplete and drop legacy 25x25 caps (#18745)

``discord_skill_commands_by_category`` was lagging the flat
``discord_skill_commands`` collector on two counts. Both were actively
dropping skills from Discord's ``/skill`` autocomplete dropdown.

1. External-dir skills were filtered out. #18741 widened the flat
   collector to accept ``SKILLS_DIR + skills.external_dirs`` but left
   this sibling collector — the one ``_register_skill_group`` actually
   uses on Discord — still matching ``SKILLS_DIR`` only. External
   skills were visible in ``hermes skills list`` and the agent's
   ``/skill-name`` dispatch but silently absent from Discord's
   ``/skill`` picker. Widen the accepted roots to match, and derive
   categories from whichever root the skill lives under so
   ``<ext>/mlops/foo/SKILL.md`` still lands in the ``mlops`` group.

2. 25-group × 25-subcommand caps were still applied. PR #11580
   refactored ``/skill`` to a flat autocomplete (whose options Discord
   fetches dynamically — no per-command payload concern) and its
   docstring promises "no hidden skills." The collector kept the old
   nested-layout caps anyway, silently dropping anything past the 25th
   alphabetical category. On installs with 29 category dirs today (real
   example: tail categories ``social-media``, ``software-development``,
   ``yuanbao`` going missing) this was biting immediately. Remove the
   caps; ``hidden`` now reports only 32-char name-clamp collisions
   against reserved names.

Tests: guard both behaviors. ``test_no_legacy_25x25_cap`` builds 30
categories × 30 skills each and asserts all 900 are returned.
``test_external_dirs_skills_included`` monkeypatches
``get_external_skills_dirs`` and asserts an external-dir skill makes
it into the result grouped under its own top-level directory.

tests/hermes_cli/test_commands.py

@@ -1420,6 +1420,119 @@ class TestDiscordSkillCommandsByCategory:
         assert "vllm" in names
         assert len(uncategorized) == 0
 
+    def test_no_legacy_25x25_cap(self, tmp_path, monkeypatch):
+        """The old nested-layout caps (25 groups × 25 skills/group) are gone.
+
+        The live caller flattens categories into a single autocomplete list,
+        which Discord fetches dynamically — the per-command 8KB payload
+        concern from the old nested layout (#11321, #10259) no longer applies.
+        Guards against accidentally re-introducing the caps, which would
+        silently drop skills in the 26th+ alphabetical category (the exact
+        failure mode users were hitting with 29 category dirs on real
+        installs).
+        """
+        from unittest.mock import patch
+
+        fake_skills_dir = str(tmp_path / "skills")
+
+        # Build 30 categories (> old _MAX_GROUPS=25) each with 30 skills
+        # (> old _MAX_PER_GROUP=25).
+        fake_cmds = {}
+        for c in range(30):
+            cat = f"cat{c:02d}"  # cat00, cat01, ..., cat29 — 30 categories
+            for s in range(30):
+                name = f"skill-{c:02d}-{s:02d}"
+                skill_subdir = tmp_path / "skills" / cat / name
+                skill_subdir.mkdir(parents=True, exist_ok=True)
+                (skill_subdir / "SKILL.md").write_text("---\nname: x\n---\n")
+                fake_cmds[f"/{name}"] = {
+                    "name": name,
+                    "description": f"Category {cat} skill {s}",
+                    "skill_md_path": f"{fake_skills_dir}/{cat}/{name}/SKILL.md",
+                }
+
+        monkeypatch.setenv("HERMES_HOME", str(tmp_path))
+        with (
+            patch("agent.skill_commands.get_skill_commands", return_value=fake_cmds),
+            patch("tools.skills_tool.SKILLS_DIR", tmp_path / "skills"),
+        ):
+            categories, uncategorized, hidden = discord_skill_commands_by_category(
+                reserved_names=set(),
+            )
+
+        # Every category should be present — no 25-group cap
+        assert len(categories) == 30, (
+            f"expected all 30 categories, got {len(categories)} "
+            f"(cap from old nested layout must be removed)"
+        )
+        # Every skill in every category must be present — no 25-per-group cap
+        for cat_name, entries in categories.items():
+            assert len(entries) == 30, (
+                f"category {cat_name}: expected 30 skills, got {len(entries)} "
+                f"(cap from old nested layout must be removed)"
+            )
+        # Nothing should be reported hidden for the cap reason (the only
+        # legitimate hidden reason now is name clamp collisions, which
+        # don't happen here since all names are unique).
+        assert hidden == 0
+
+    def test_external_dirs_skills_included(self, tmp_path, monkeypatch):
+        """Skills in ``skills.external_dirs`` must appear in /skill autocomplete.
+
+        #18741 fixed this for the flat ``discord_skill_commands`` collector
+        but left ``discord_skill_commands_by_category`` (the live caller for
+        Discord's ``/skill`` command) still filtering by
+        ``SKILLS_DIR`` prefix only. Regression guard that both collectors
+        now accept external-dir skills.
+        """
+        from unittest.mock import patch
+
+        local_skills_dir = tmp_path / "local-skills"
+        external_dir = tmp_path / "external-skills"
+
+        (local_skills_dir / "creative" / "local-skill").mkdir(parents=True)
+        (local_skills_dir / "creative" / "local-skill" / "SKILL.md").write_text("")
+
+        (external_dir / "mlops" / "external-skill").mkdir(parents=True)
+        (external_dir / "mlops" / "external-skill" / "SKILL.md").write_text("")
+
+        fake_cmds = {
+            "/local-skill": {
+                "name": "local-skill",
+                "description": "Local",
+                "skill_md_path": str(local_skills_dir / "creative" / "local-skill" / "SKILL.md"),
+            },
+            "/external-skill": {
+                "name": "external-skill",
+                "description": "External",
+                "skill_md_path": str(external_dir / "mlops" / "external-skill" / "SKILL.md"),
+            },
+        }
+        monkeypatch.setenv("HERMES_HOME", str(tmp_path))
+        with (
+            patch("agent.skill_commands.get_skill_commands", return_value=fake_cmds),
+            patch("tools.skills_tool.SKILLS_DIR", local_skills_dir),
+            patch(
+                "agent.skill_utils.get_external_skills_dirs",
+                return_value=[external_dir],
+            ),
+        ):
+            categories, uncategorized, hidden = discord_skill_commands_by_category(
+                reserved_names=set(),
+            )
+
+        # Local skill → grouped under "creative"
+        assert "creative" in categories
+        assert any(n == "local-skill" for n, _d, _k in categories["creative"])
+        # External skill → grouped under its own top-level dir "mlops"
+        assert "mlops" in categories, (
+            "external-dir skills must be included — the old SKILLS_DIR-only "
+            "prefix check was broken for by_category (completes #18741)"
+        )
+        assert any(n == "external-skill" for n, _d, _k in categories["mlops"])
+        assert uncategorized == []
+        assert hidden == 0
+
 
 # ---------------------------------------------------------------------------
 # Plugin slash command integration