feat: Add Docker-based preview deployments for phase review

Preview deployments let reviewers spin up the app at a specific branch
in local Docker containers, accessible through a single Caddy reverse
proxy port. Docker is the source of truth — no database table needed.

New module: src/preview/ with config discovery (.cw-preview.yml →
compose → Dockerfile fallback), compose generation, Docker CLI wrapper,
health checking, and port allocation (9100-9200 range).

docs/dispatch-events.md

@@ -11,7 +11,7 @@
 - **Adapter**: `TypedEventBus` using Node.js `EventEmitter`
 - All events implement `BaseEvent { type, timestamp, payload }`
 
-### Event Types (48)
+### Event Types (52)
 
 | Category | Events | Count |
 |----------|--------|-------|
@@ -24,6 +24,7 @@
 | **Server** | `server:started`, `server:stopped` | 2 |
 | **Worktree** | `worktree:created`, `worktree:removed`, `worktree:merged`, `worktree:conflict` | 4 |
 | **Account** | `account:credentials_refreshed`, `account:credentials_expired`, `account:credentials_validated` | 3 |
+| **Preview** | `preview:building`, `preview:ready`, `preview:stopped`, `preview:failed` | 4 |
 | **Log** | `log:entry` | 1 |
 
 ### Key Event Payloads