feat(server): ASGI endpoint via uvicorn

Implement FastAPI-based ASGI application served by uvicorn for
the CleverAgents server mode. Add health check endpoint, A2A
JSON-RPC routing, configurable host:port binding, and graceful
shutdown handling. Server launches via `agents server start`.

- Add FastAPI ASGI app factory at infrastructure/server/asgi_app.py
  with /.well-known/agent.json (Agent Card), /health (liveness),
  and /a2a (A2A JSON-RPC 2.0 dispatch via A2aLocalFacade)
- Add ServerLifecycle class at infrastructure/server/server_lifecycle.py
  wrapping uvicorn.Server with SIGTERM/SIGINT graceful shutdown
- Add `agents server start` CLI command with --host, --port, --log-level
  options, resolving defaults from Settings
- Add fastapi>=0.115.0 dependency to pyproject.toml (uvicorn already present)
- Add Behave BDD tests (features/server_lifecycle.feature, 20 scenarios)
- Add Robot Framework integration tests (robot/server_lifecycle.robot)
- Update CHANGELOG.md and vulture_whitelist.py

ISSUES CLOSED: #862

src/cleveragents/cli/main.py

@@ -223,7 +223,7 @@ def _register_subcommands() -> None:
     app.add_typer(
         server_app,
         name="server",
-        help="Server connection management (stub)",
+        help="Server management — start, connect, and status",
     )
     app.add_typer(
         repo.app,