feat(coding-agent/eval): added local python-runner subprocess execution

- Replaced Python execution with a local `python -u runner.py` subprocess and NDJSON stdin/stdout framing.
- Removed shared-gateway architecture, including coordinator lifecycle APIs, `useSharedGateway` wiring, and `jupyter` CLI/actions.
- Simplified setup checks to a plain Python 3 availability probe and removed automatic dependency-install fallbacks.
- Updated kernel cancellation and display processing to use status frames, SIGINT/SIGTERM escalation, and normalized output coercion.
- Added `python-runner` integration and display tests while deleting legacy websocket and kernel lifecycle test suites.

packages/coding-agent/src/session/agent-session.ts

@@ -6561,7 +6561,6 @@ export class AgentSession {
 				sessionId,
 				kernelOwnerId: this.#evalKernelOwnerId,
 				kernelMode: this.settings.get("python.kernelMode"),
-				useSharedGateway: this.settings.get("python.sharedGateway"),
 				onChunk,
 				signal: abortController.signal,
 			});