Navi Code: stop via Esc + project cwd propagation ...

- TUI: Esc stops active stream cooperatively via POST /sessions/{id}/stop
- TUI: render stream_stopped as status message
- CLI/WebSocket: send shell cwd in client->server message field
- Orchestrator stores cwd in session.session_metadata
- ContextBuilder injects [Working directory] into LLM context
- Agent sets current_working_directory ContextVar per turn
- tools/base: ToolContext gains cwd field
- filesystem/terminal/code_exec resolve relative paths against session cwd
- Add bin/navi-code wrapper for PATH symlink; document in README.md
- Update docs/websocket.md and tests

Full pytest: 544 passed, 1 skipped.

Co-Authored-By: Claude <noreply@anthropic.com>