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/settings.py

@@ -0,0 +1,56 @@
+import json
+import os
+
+
+def _get_bool(name, default=False):
+    val = os.environ.get(name)
+    if val is None:
+        return default
+    return val.strip().lower() in ("1", "true", "yes", "on")
+
+
+def _get_json_list(name, default=None):
+    raw = os.environ.get(name)
+    if not raw:
+        return list(default or [])
+    try:
+        value = json.loads(raw)
+    except json.JSONDecodeError:
+        raise ValueError(f"{name} must be a JSON array string")
+    if not isinstance(value, list):
+        raise ValueError(f"{name} must be a JSON array")
+    return value
+
+
+def _get_int(name, default):
+    raw = os.environ.get(name)
+    if raw is None:
+        return default
+    return int(raw)
+
+
+DATA_DIR = os.environ.get("CROSS_PY_DATA", os.path.join(os.path.dirname(os.path.dirname(__file__)), "data"))
+DB_PATH = os.environ.get("CROSS_PY_DB", os.path.join(DATA_DIR, "jobs.db"))
+BUILDS_DIR = os.environ.get("CROSS_PY_BUILDS", os.path.join(DATA_DIR, "builds"))
+PORT = _get_int("CROSS_PY_PORT", 8080)
+
+# Static worker list: JSON array of "host:port" strings.
+WORKERS = _get_json_list("CROSS_PY_WORKERS")
+
+# One job at a time by default; raise to allow parallel dispatches to distinct workers.
+MAX_CONCURRENT = _get_int("CROSS_PY_MAX_CONCURRENT", 1)
+
+# Per-request timeouts (seconds) for talking to workers.
+WORKER_STATUS_TIMEOUT = _get_int("CROSS_PY_WORKER_STATUS_TIMEOUT", 5)
+WORKER_BUILD_TIMEOUT = _get_int("CROSS_PY_WORKER_BUILD_TIMEOUT", 3600)
+
+# Interval (seconds) for the scheduler loop.
+SCHEDULER_INTERVAL = 1.0
+
+# Debug mode uses Flask's dev server; otherwise serve through waitress.
+DEBUG = _get_bool("CROSS_PY_DEBUG", False)
+
+
+def ensure_dirs():
+    os.makedirs(DATA_DIR, exist_ok=True)
+    os.makedirs(BUILDS_DIR, exist_ok=True)