feat: interim identifier registrar (RMGR-WP-0005-T01)

Add is_identifier_registrar so only the production instance may mint
hub UUIDs into repository files. Workstation hubs stay read/project
caches until deterministic derivation lands.

Also close RMGR-WP-0004-T05: register cutover is STATE-WP-0080-T02.

src/repo_manager/registrar.py

@@ -0,0 +1,30 @@
+"""Interim single-writer for hub identifiers (ADR-007 decision 2).
+
+Until deterministic derivation (RMGR-WP-0005-T03) ships, exactly one
+instance may mint ``state_hub_workstream_id`` / ``state_hub_task_id``
+into repository files. Other instances may read and project; they must
+not write new hub primary keys into git-tracked files.
+
+The registrar is the automated production instance (hostname prefix
+``railiance``). Override with ``STATEHUB_REGISTRAR=1`` or ``=0``.
+"""
+
+from __future__ import annotations
+
+import os
+import socket
+
+_TRUE = frozenset({"1", "true", "yes", "on"})
+_FALSE = frozenset({"0", "false", "no", "off"})
+REGISTRAR_HOST_PREFIX = "railiance"
+
+
+def is_identifier_registrar(*, hostname: str | None = None) -> bool:
+    """Return True if this process may mint hub IDs into repository files."""
+    raw = os.environ.get("STATEHUB_REGISTRAR", "").strip().lower()
+    if raw in _TRUE:
+        return True
+    if raw in _FALSE:
+        return False
+    host = (hostname if hostname is not None else socket.gethostname()).strip().lower()
+    return host.startswith(REGISTRAR_HOST_PREFIX)