Response to teknium1's hermes-sweeper review (keep_open, salvageability=medium).
1. Post-auth gate. The group-99 catch-all fired gateway_platform_event before
the authorization boundary. Extract _is_source_authorized(source) from
_is_user_authorized_from_message and add _source_from_reaction_for_auth;
reactions whose actor the intake would reject no longer reach plugins.
Fails closed if source extraction raises, so a future non-reaction event
type cannot silently bypass auth before its own extraction is wired.
2. Shared registration. Extract _register_handlers(app) from connect() so the
gateway_platform_event observer (group 99) is re-registered alongside the
core handlers on any rebuild path.
3. Trim inert hook surface. Drop the three reserved gateway_* names from
VALID_HOOKS (keep only gateway_platform_event). The others land with their
real contracts and fire-sites when #64231 is finalized.
Tests: unauthorized/authorized/open reaction gating, fail-closed for a future
non-reaction event type, and _register_handlers re-registration.
Ran /simplify and /code-review (high) before pushing.
Paolo Antinori
c0a4535a26