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.

ctrl/app.py

@@ -0,0 +1,158 @@
+import datetime
+import json
+import os
+import queue
+
+from flask import Flask, jsonify, request, send_from_directory, Response
+
+import ctrl.db as db
+import ctrl.settings as settings
+import ctrl.workers as wrk
+from ctrl.scheduler import make_scheduler
+
+app = Flask(__name__, static_folder="static", static_url_path="")
+scheduler = make_scheduler()
+
+VIEW_STATIC = os.path.join(os.path.dirname(__file__), "static")
+
+_initialized = False
+
+
+def _ensure_ready():
+    global _initialized
+    if _initialized:
+        return
+    settings.ensure_dirs()
+    db.init_db()
+    _initialized = True
+
+
+@app.before_request
+def _before_request():
+    _ensure_ready()
+
+
+@app.route("/")
+def index():
+    return send_from_directory(VIEW_STATIC, "index.html")
+
+
+@app.get("/api/workers")
+def api_workers():
+    return jsonify(wrk.probe_all())
+
+
+@app.get("/api/capabilities")
+def api_capabilities():
+    combos = {
+        (w.get("os"), w.get("cpu"))
+        for w in wrk.probe_all()
+        if w.get("os") and w.get("cpu")
+    }
+    return jsonify(sorted({"os": o, "cpu": c} for o, c in combos if o and c))
+
+
+@app.post("/api/jobs")
+def api_create_job():
+    os_req = (request.form.get("os") or "").strip() or None
+    cpu_req = (request.form.get("cpu") or "").strip() or None
+
+    if request.files and "file" in request.files:
+        upload = request.files["file"]
+        if not upload.filename:
+            return jsonify({"error": "No file selected"}), 400
+        source_dir = os.path.join(settings.DATA_DIR, "sources")
+        os.makedirs(source_dir, exist_ok=True)
+        fname = os.path.basename(upload.filename)
+        dest = os.path.join(source_dir, fname)
+        upload.save(dest)
+        job_id = db.create_job("upload", fname, os_req, cpu_req)
+    else:
+        data = request.get_json(silent=True) or {}
+        repo_url = (data.get("repo_url") or "").strip()
+        if not repo_url:
+            return jsonify({"error": "Provide a file upload or a repo_url"}), 400
+        job_id = db.create_job("git", repo_url, os_req, cpu_req)
+
+    scheduler.start()
+    return jsonify({"id": job_id}), 201
+
+
+@app.get("/api/jobs")
+def api_list_jobs():
+    return jsonify(db.list_jobs())
+
+
+@app.get("/api/jobs/<job_id>")
+def api_get_job(job_id):
+    job = db.get_job(job_id)
+    if not job:
+        return jsonify({"error": "not found"}), 404
+    return jsonify(job)
+
+
+@app.get("/api/jobs/<job_id>/artifacts/<name>")
+def api_download(job_id, name):
+    job = db.get_job(job_id)
+    if not job:
+        return jsonify({"error": "not found"}), 404
+    safe = os.path.basename(name)
+    out_dir = os.path.join(settings.BUILDS_DIR, job_id)
+    return send_from_directory(out_dir, safe, as_attachment=True)
+
+
+@app.post("/api/jobs/<job_id>/cancel")
+def api_cancel(job_id):
+    job = db.get_job(job_id)
+    if not job:
+        return jsonify({"error": "not found"}), 404
+    if job["status"] in ("queued",):
+        db.update_job(job_id, status="cancelled", finished_at=datetime.datetime.now().isoformat())
+        return jsonify({"id": job_id, "status": "cancelled"})
+    return jsonify({"error": f"Cannot cancel job in state {job['status']}"}), 400
+
+
+@app.get("/api/jobs/<job_id>/stream")
+def api_stream(job_id):
+    job = db.get_job(job_id)
+    if not job:
+        return jsonify({"error": "not found"}), 404
+
+    q = scheduler.subscribe(job_id)
+
+    def gen():
+        # initial snapshot
+        snap = db.get_job(job_id)
+        if snap:
+            yield f"event: job\ndata: {json.dumps({'status': snap['status']})}\n\n"
+            for line in (snap.get("log") or "").splitlines():
+                yield f"event: log\ndata: {json.dumps(line)}\n\n"
+        try:
+            while True:
+                try:
+                    frame = q.get(timeout=15)
+                    yield frame
+                except queue.Empty:
+                    # heartbeat to keep connection alive
+                    yield ": keepalive\n\n"
+                job_now = db.get_job(job_id)
+                if job_now and job_now["status"] in ("done", "failed", "cancelled"):
+                    break
+        finally:
+            scheduler.unsubscribe(job_id, q)
+
+    return Response(gen(), mimetype="text/event-stream")
+
+
+def main():
+    _ensure_ready()
+    scheduler.start()
+    if settings.DEBUG:
+        app.run(host="0.0.0.0", port=settings.PORT, threaded=True)
+        return
+    from waitress import serve
+    serve(app, host="0.0.0.0", port=settings.PORT, threads=8)
+
+
+if __name__ == "__main__":
+    main()