feat(RMGR-WP-0002): dual-run writeback, flags, meter, SH facade

Add dual-run flags/meter, harden task-status (idempotency, UUID, head,
push), State Hub adapter for PATCH /tasks and C-15/reconcile proxy, pilot
evidence, and finish RMGR-WP-0002.

src/repo_manager/idempotency.py

@@ -0,0 +1,45 @@
+"""Simple file-backed idempotency for dual-run commands."""
+
+from __future__ import annotations
+
+import hashlib
+import json
+import os
+from pathlib import Path
+from typing import Any
+
+
+def _store_path() -> Path:
+    raw = os.environ.get("RM_IDEMPOTENCY_PATH", "~/.repo-manager/idempotency.json")
+    return Path(raw).expanduser()
+
+
+def _load() -> dict[str, Any]:
+    path = _store_path()
+    if not path.is_file():
+        return {}
+    try:
+        return json.loads(path.read_text(encoding="utf-8"))
+    except (OSError, json.JSONDecodeError):
+        return {}
+
+
+def _save(data: dict[str, Any]) -> None:
+    path = _store_path()
+    path.parent.mkdir(parents=True, exist_ok=True)
+    path.write_text(json.dumps(data, indent=2) + "\n", encoding="utf-8")
+
+
+def payload_hash(payload: dict[str, Any]) -> str:
+    blob = json.dumps(payload, sort_keys=True, default=str)
+    return hashlib.sha256(blob.encode()).hexdigest()
+
+
+def get(key: str) -> dict[str, Any] | None:
+    return _load().get(key)
+
+
+def put(key: str, payload_hash_value: str, result: dict[str, Any]) -> None:
+    data = _load()
+    data[key] = {"payload_hash": payload_hash_value, "result": result}
+    _save(data)