feat: finish register receiving and authority routing

Assistant: codex
Assistant-Model: gpt-5.6-sol
Assistant-Session: 01a023c0-a0a3-7c03-b395-5a0d2757214d

src/repo_manager/index_store.py

@@ -4,13 +4,14 @@ from __future__ import annotations
 
 import json
 from dataclasses import asdict, dataclass, field
-from datetime import datetime, timezone
 from pathlib import Path
 from typing import Any
 
+from repo_manager.time import utc_now_text
+
 
 def _now() -> str:
-    return datetime.now(timezone.utc).isoformat()
+    return utc_now_text()
 
 
 @dataclass