feat: tool definition manifest system (PR 1)

- Add ToolDefinitionManifest model with base image versioning
- Add manifest compiler: Dockerfile + Compose generation from JSON manifests
- Add permission fixer: post-start chown/chmod for mount policies
- Add tool definition CRUD API with live compile preview endpoint
- Integrate manifest-based startup flow in start_instance
- Add Alembic migration with data conversion for pi-agent
- Add 48 unit tests for manifest compiler, permission fixer, docker service
- Keep backward compatibility with legacy dockerfile_template/compose_template

Migration: applied successfully. Pi-agent converted to manifest.
Quality gates: pytest (146 passed, 4 pre-existing unrelated failures)

.pi/settings.json

@@ -0,0 +1 @@
+{}