feat: Replace per-preview Caddy sidecars with shared gateway architecture

Refactor preview deployments to use a single shared Caddy gateway container
with subdomain routing (<previewId>.localhost:<port>) instead of one Caddy
sidecar and one port per preview. Adds dev/preview modes, git worktree
support for branch checkouts, and auto-start on phase:pending_review.

- Add GatewayManager for shared Caddy lifecycle + Caddyfile generation
- Add git worktree helpers for preview mode branch checkouts
- Add dev mode: volume-mount + dev server image instead of build
- Remove per-preview Caddy sidecar and port publishing
- Use shared cw-preview-net Docker network with container name DNS
- Auto-start previews when phase enters pending_review
- Delete unused PreviewPanel.tsx
- Update all tests (40 pass), docs, events, CLI, tRPC, frontend

apps/server/trpc/routers/preview.ts

@@ -14,6 +14,8 @@ export function previewProcedures(publicProcedure: ProcedureBuilder) {
         phaseId: z.string().min(1).optional(),
         projectId: z.string().min(1),
         branch: z.string().min(1),
+        mode: z.enum(['preview', 'dev']).default('preview'),
+        worktreePath: z.string().optional(),
       }))
       .mutation(async ({ ctx, input }) => {
         const previewManager = requirePreviewManager(ctx);