feat: finish register receiving and authority routing

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

tests/test_time.py

@@ -0,0 +1,17 @@
+from datetime import UTC, datetime, timedelta, timezone
+
+import pytest
+
+from repo_manager.time import format_utc
+
+
+def test_format_utc_normalizes_offsets_and_uses_z() -> None:
+    local = datetime(2026, 8, 21, 22, 30, tzinfo=timezone(timedelta(hours=2)))
+
+    assert format_utc(local) == "2026-08-21T20:30:00Z"
+    assert format_utc(local.astimezone(UTC)).endswith("Z")
+
+
+def test_format_utc_rejects_naive_datetime() -> None:
+    with pytest.raises(ValueError, match="timezone-aware"):
+        format_utc(datetime(2026, 8, 21, 20, 30))  # noqa: DTZ001 - deliberate invalid input