Add controller with web UI and retire CLI

- Add ctrl/ package: Flask app (waitress), SQLite job store, background
  scheduler that probes configured workers, dispatches via /upload and
  /checkout_git, fetches artifacts, and serves a single-page UI
- Supporting API: /api/jobs (create/list/detail/cancel), /api/workers,
  /api/capabilities, artifact download, and SSE log stream
- Workers keep the existing HTTP API; add /progress/<job_id> endpoint and
  per-step build tracking for live status
- Retire agent_manager.py and the cross-py-builder CLI; controller UI is
  the primary frontend
- Record design in DESIGN.md; add waitress dependency

Note: /progress is exposed on the worker but the synchronous /upload and
/checkout endpoints block until a build completes, so per-step worker
progress is not yet relayed live in the UI. Live streaming needs an
async-start worker endpoint as a follow-up.

setup.py

@@ -8,7 +8,7 @@ setup(
     entry_points={
         "console_scripts": [
             "cross-py-agent=cross_py_builder.build_agent:main",
-            "cross-py-builder=cross_py_builder.agent_manager:main",
+            "cross-py-controller=ctrl.app:main",
         ],
     },
     author="Brett Williams",