feat: add Slack channel adapter (Phase 3b)

Implement ChannelAdapter for Slack using @slack/bolt with Socket Mode:
- Thread-aware peer IDs (channelId:threadTs)
- Bot message and channel allowlist filtering
- Bot mention stripping (<@U\w+> pattern)
- Message chunking at 4000 chars for readability
- Error handling in connect/disconnect lifecycle
- Typed SlackMessageEvent interface
- 22 tests covering all behaviors

src/channels/slack/index.ts

@@ -0,0 +1 @@
+export { SlackAdapter, type SlackAdapterConfig } from './adapter.js';