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
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