/add-slack (#366)

* feat: add Slack channel skill (/add-slack)

Slack Bot integration via @slack/bolt with Socket Mode. Can replace
WhatsApp entirely (SLACK_ONLY=true) or run alongside it.

- SlackChannel implementing Channel interface (46 unit tests)
- Socket Mode connection (no public URL needed)
- @mention translation (Slack <@UBOTID> → TRIGGER_PATTERN)
- Message splitting at 4000-char Slack API limit
- Thread flattening (threaded replies delivered as channel messages)
- User name resolution with caching
- Outgoing message queue with flush-on-reconnect
- Channel metadata sync with pagination
- Proper Bolt types (GenericMessageEvent | BotMessageEvent)
- Multi-channel orchestrator changes (conditional channel creation)
- Setup guide (SLACK_SETUP.md) and known limitations documented

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* local settings

* adjusted when installing

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

.claude/skills/add-slack/modify/src/routing.test.ts.intent.md

@@ -0,0 +1,17 @@
+# Intent: src/routing.test.ts modifications
+
+## What changed
+Added Slack JID pattern tests and Slack-specific getAvailableGroups tests.
+
+## Key sections
+- **JID ownership patterns**: Added Slack channel JID (`slack:C...`) and Slack DM JID (`slack:D...`) pattern tests
+- **getAvailableGroups**: Added tests for Slack channel inclusion, Slack DM handling, registered Slack channels, and mixed WhatsApp + Slack ordering
+
+## Invariants
+- All existing WhatsApp JID pattern tests remain unchanged
+- All existing getAvailableGroups tests remain unchanged
+- New tests follow the same patterns as existing tests
+
+## Must-keep
+- All existing WhatsApp tests (group JID, DM JID patterns)
+- All existing getAvailableGroups tests (DM exclusion, sentinel exclusion, registration, ordering, non-group exclusion, empty array)