…d WebSocket connectivity

Adds device OAuth authentication, a headless `serve` command with WebSocket connectivity
via Pusher protocol, workspace management, Claude session management over WebSocket,
and a self-update mechanism. The existing TUI experience is preserved by refactoring the
loop engine into a shared core that both TUI and serve consume.

Key changes:
- Migrate CLI to Cobra framework for structured command tree
- Add login/logout commands with device OAuth flow
- Add serve command for headless WebSocket-connected operation
- Add uplink package (Pusher client, HTTP client, message batcher)
- Add engine package as shared loop abstraction
- Add workspace scanner and file watcher
- Add WebSocket message types and rate limiting
- Remove multi-agent support (Claude-only going forward)
- Remove markdown PRD format (JSON-only)
- Update docs to reflect new architecture

Reverts docs/, CONTRIBUTING.md, LICENSE, and internal/update/update.go
changes that were unrelated to the uplink feature.