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.
