Add Pi bridge integration

[mmprotest]

Motivation

• Provide a thin, first-class Pi integration so Pi can delegate runs to Villani without porting Villani runner logic into Pi.
• Define a stable, minimal JSONL stdio protocol so external integrations (Pi) can start/observe/abort Villani runs and receive normalized operational events.

Description

• Add a small JSON bridge protocol with typed dataclasses and parsing in villani_code/integrations/pi_bridge_protocol.py implementing ready, ping, run, and abort semantics.
• Implement a threaded stdio bridge (PiBridge) in villani_code/integrations/pi_bridge.py that emits flushed JSONL events, spawns the existing Runner via _build_runner, maps internal runner events to normalized bridge events, detects changed files via git status, and propagates transcript path/verification results.
• Expose a CLI entrypoint bridge --stdio in villani_code/cli.py that runs the stdio bridge so it can be launched as python -m villani_code.cli bridge --stdio (or via an executable).
• Add a Pi extension scaffold under integrations/pi-villani/ (TypeScript) which registers a /villani command, launches the bridge subprocess using VILLANI_COMMAND, waits for ready, sends a run command, streams & renders normalized bridge events, supports aborts, and produces a final summary; protocol/types and a small process wrapper live in src/protocol.ts and src/process.ts.
• Add tests tests/integrations/test_pi_bridge.py covering ping/pong, malformed JSON handling, unknown commands, mocked run success/failure, abort behavior, and event filtering; add documentation and a short design note for future Pi-backed model-proxy work (integrations/pi-villani/README.md and docs/pi-model-bridge.md).

Testing

• Ran linters: ruff check villani_code/integrations tests/integrations/test_pi_bridge.py villani_code/cli.py and it passed.
• Ran unit/integration tests: pytest -q tests/integrations/test_pi_bridge.py tests/test_packaging_surface.py tests/test_cli_headless_optional_tui.py and all tests passed (19 passed, 1 skipped in the broader run; bridge tests: 7 passed).
• Built the Pi extension: cd integrations/pi-villani && npm install && npm run build and the TypeScript build succeeded.
• Performed a manual smoke test: printf '{"type":"ping","id":"smoke"}\n' | python -m villani_code.cli bridge --stdio produced the expected ready and pong JSON lines.

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Codex Task