refactor(gateway): extract WhatsApp identity helpers into shared module

Follow-up to the canonical-identity session-key fix: pull the
JID/LID normalize/expand/canonical helpers into gateway/whatsapp_identity.py
instead of living in two places. gateway/session.py (session-key build) and
gateway/run.py (authorisation allowlist) now both import from the shared
module, so the two resolution paths can't drift apart.

Also switches the auth path from module-level _hermes_home (cached at
import time) to dynamic get_hermes_home() lookup, which matches the
session-key path and correctly reflects HERMES_HOME env overrides. The
lone test that monkeypatched gateway.run._hermes_home for the WhatsApp
auth path is updated to set HERMES_HOME env var instead; all other
tests that monkeypatch _hermes_home for unrelated paths (update,
restart drain, shutdown marker, etc.) still work — the module-level
_hermes_home is untouched.

gateway/run.py

@@ -298,50 +298,16 @@ from gateway.restart import (
 )
 
 
-def _normalize_whatsapp_identifier(value: str) -> str:
-    """Strip WhatsApp JID/LID syntax down to its stable numeric identifier."""
-    return (
-        str(value or "")
-        .strip()
-        .replace("+", "", 1)
-        .split(":", 1)[0]
-        .split("@", 1)[0]
-    )
+from gateway.whatsapp_identity import (
+    canonical_whatsapp_identifier as _canonical_whatsapp_identifier,  # noqa: F401
+    expand_whatsapp_aliases as _expand_whatsapp_auth_aliases,
+    normalize_whatsapp_identifier as _normalize_whatsapp_identifier,
+)
 
 
-def _expand_whatsapp_auth_aliases(identifier: str) -> set:
-    """Resolve WhatsApp phone/LID aliases using bridge session mapping files."""
-    normalized = _normalize_whatsapp_identifier(identifier)
-    if not normalized:
-        return set()
-
-    session_dir = _hermes_home / "whatsapp" / "session"
-    resolved = set()
-    queue = [normalized]
-
-    while queue:
-        current = queue.pop(0)
-        if not current or current in resolved:
-            continue
-
-        resolved.add(current)
-        for suffix in ("", "_reverse"):
-            mapping_path = session_dir / f"lid-mapping-{current}{suffix}.json"
-            if not mapping_path.exists():
-                continue
-            try:
-                mapped = _normalize_whatsapp_identifier(
-                    json.loads(mapping_path.read_text(encoding="utf-8"))
-                )
-            except Exception:
-                continue
-            if mapped and mapped not in resolved:
-                queue.append(mapped)
-
-    return resolved
-
 logger = logging.getLogger(__name__)
 
+
 # Sentinel placed into _running_agents immediately when a session starts
 # processing, *before* any await.  Prevents a second message for the same
 # session from bypassing the "already running" guard during the async gap