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/architecture.md

@@ -20,7 +20,8 @@ CLI (cw)
         │     └── LifecycleController (retry, signal recovery)
         ├── DispatchManager (task queue, dependency resolution)
         ├── PhaseDispatchManager (phase queue, DAG ordering)
-        └── CoordinationManager (merge queue, conflict resolution)
+        ├── CoordinationManager (merge queue, conflict resolution)
+        └── PreviewManager (Docker-based preview deployments)
 
 Web UI (packages/web/)
   └── React 19 + TanStack Router + tRPC React Query
@@ -66,6 +67,7 @@ Agent providers (Claude, Codex, etc.) are defined as configuration objects, not
 | Logging | `src/logger/`, `src/logging/` | Structured logging, file capture | [git-process-logging.md](git-process-logging.md) |
 | Events | `src/events/` | EventBus, typed event system | [dispatch-events.md](dispatch-events.md) |
 | Shared | `packages/shared/` | Types shared between frontend/backend | [frontend.md](frontend.md) |
+| Preview | `src/preview/` | Docker-based preview deployments | [preview.md](preview.md) |
 | Tests | `src/test/` | E2E, integration, fixtures | [testing.md](testing.md) |
 
 ## Entity Relationships