/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/manifest.yaml

@@ -0,0 +1,21 @@
+skill: slack
+version: 1.0.0
+description: "Slack Bot integration via @slack/bolt with Socket Mode"
+core_version: 0.1.0
+adds:
+  - src/channels/slack.ts
+  - src/channels/slack.test.ts
+modifies:
+  - src/index.ts
+  - src/config.ts
+  - src/routing.test.ts
+structured:
+  npm_dependencies:
+    "@slack/bolt": "^4.6.0"
+  env_additions:
+    - SLACK_BOT_TOKEN
+    - SLACK_APP_TOKEN
+    - SLACK_ONLY
+conflicts: []
+depends: []
+test: "npx vitest run src/channels/slack.test.ts"