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"]

tests/gateway/test_slack_channel_skills.py

@@ -0,0 +1,133 @@
+"""Tests for Slack channel_skill_bindings auto-skill resolution."""
+from unittest.mock import MagicMock
+
+
+def _make_adapter(extra=None):
+    """Create a minimal SlackAdapter stub with the given ``config.extra``."""
+    from gateway.platforms.slack import SlackAdapter
+    adapter = object.__new__(SlackAdapter)
+    adapter.config = MagicMock()
+    adapter.config.extra = extra or {}
+    return adapter
+
+
+def _resolve(adapter, channel_id, parent_id=None):
+    from gateway.platforms.base import resolve_channel_skills
+    return resolve_channel_skills(adapter.config.extra, channel_id, parent_id)
+
+
+class TestSlackResolveChannelSkills:
+    def test_no_bindings_returns_none(self):
+        adapter = _make_adapter()
+        assert _resolve(adapter, "D0ABC") is None
+
+    def test_match_by_dm_channel_id(self):
+        """The primary use case: binding a skill to a Slack DM channel."""
+        adapter = _make_adapter({
+            "channel_skill_bindings": [
+                {"id": "D0ATH9TQ0G6", "skills": ["german-flashcards"]},
+            ]
+        })
+        assert _resolve(adapter, "D0ATH9TQ0G6") == ["german-flashcards"]
+
+    def test_match_by_parent_id_for_thread(self):
+        """Slack threads inherit the parent channel's binding."""
+        adapter = _make_adapter({
+            "channel_skill_bindings": [
+                {"id": "C0PARENT", "skills": ["parent-skill"]},
+            ]
+        })
+        assert _resolve(adapter, "thread-ts-123", parent_id="C0PARENT") == ["parent-skill"]
+
+    def test_no_match_returns_none(self):
+        adapter = _make_adapter({
+            "channel_skill_bindings": [
+                {"id": "D0AAA", "skills": ["skill-a"]},
+            ]
+        })
+        assert _resolve(adapter, "D0BBB") is None
+
+    def test_single_skill_string(self):
+        adapter = _make_adapter({
+            "channel_skill_bindings": [
+                {"id": "D0ATH9TQ0G6", "skill": "german-flashcards"},
+            ]
+        })
+        assert _resolve(adapter, "D0ATH9TQ0G6") == ["german-flashcards"]
+
+    def test_dedup_preserves_order(self):
+        adapter = _make_adapter({
+            "channel_skill_bindings": [
+                {"id": "D0ATH9TQ0G6", "skills": ["a", "b", "a", "c", "b"]},
+            ]
+        })
+        assert _resolve(adapter, "D0ATH9TQ0G6") == ["a", "b", "c"]
+
+    def test_multiple_bindings_pick_correct(self):
+        adapter = _make_adapter({
+            "channel_skill_bindings": [
+                {"id": "D0AAA", "skills": ["skill-a"]},
+                {"id": "D0BBB", "skills": ["skill-b"]},
+                {"id": "D0CCC", "skills": ["skill-c"]},
+            ]
+        })
+        assert _resolve(adapter, "D0BBB") == ["skill-b"]
+
+    def test_malformed_entry_skipped(self):
+        """Non-dict entries should be ignored, not raise."""
+        adapter = _make_adapter({
+            "channel_skill_bindings": [
+                "not-a-dict",
+                {"id": "D0ABC", "skills": ["good"]},
+            ]
+        })
+        assert _resolve(adapter, "D0ABC") == ["good"]
+
+    def test_empty_skills_list_returns_none(self):
+        adapter = _make_adapter({
+            "channel_skill_bindings": [
+                {"id": "D0ABC", "skills": []},
+            ]
+        })
+        assert _resolve(adapter, "D0ABC") is None
+
+    def test_empty_skill_string_returns_none(self):
+        adapter = _make_adapter({
+            "channel_skill_bindings": [
+                {"id": "D0ABC", "skill": ""},
+            ]
+        })
+        assert _resolve(adapter, "D0ABC") is None
+
+
+class TestSlackMessageEventAutoSkill:
+    """Integration-style test: verify auto_skill propagates to MessageEvent."""
+
+    def test_message_event_carries_auto_skill(self):
+        """Simulate the handler wiring: resolve + attach to MessageEvent."""
+        from gateway.platforms.base import MessageEvent, MessageType, Platform, SessionSource, resolve_channel_skills
+
+        config_extra = {
+            "channel_skill_bindings": [
+                {"id": "D0ATH9TQ0G6", "skills": ["german-flashcards"]},
+            ]
+        }
+        auto_skill = resolve_channel_skills(config_extra, "D0ATH9TQ0G6", None)
+
+        source = SessionSource(
+            platform=Platform.SLACK,
+            chat_id="D0ATH9TQ0G6",
+            chat_name="Mats",
+            chat_type="dm",
+            user_id="U0ABC",
+            user_name="Mats",
+        )
+        event = MessageEvent(
+            text="work",
+            message_type=MessageType.TEXT,
+            source=source,
+            raw_message={},
+            message_id="123.456",
+            auto_skill=auto_skill,
+        )
+        assert event.auto_skill == ["german-flashcards"]