Cherry-pick: extensions-move Slack (252 files)

## Extensions-move: Slack adapter

**Upstream commit**: `8746362f5e`
**Files**: ~252
**Decomposed from**: #2018

### Conflict Resolution
Move `src/slack/` → `extensions/slack/src/`. Keep fork's rebranded content.

### Exit Criteria
- `pnpm typecheck` passes
- No `openclaw` references in moved files