/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/settings.local.json

@@ -0,0 +1,18 @@
+{
+  "permissions": {
+    "allow": [
+      "Bash(grep:*)",
+      "Bash(npm run build:*)",
+      "Bash(npm test:*)",
+      "Bash(git checkout:*)",
+      "Bash(npm install:*)",
+      "Bash(npx vitest run:*)",
+      "Bash(npx tsx:*)",
+      "Bash(node:*)",
+      "Bash(npm pack:*)",
+      "Bash(git status:*)",
+      "Bash(git add:*)",
+      "Bash(git commit:*)"
+    ]
+  }
+}