fix(matrix): isolate room context and restore reliable inbound dispatch (#18505)

* fix(matrix): isolate room context and inbound dispatch

* test(matrix): cover room isolation and dispatch regressions

* docs(matrix): document room isolation and session scope

* fix(matrix): stabilize CI requirement checks

* test(matrix): isolate mautrix stubs in requirements tests

* fix(matrix): port room-scoped status and resume to slash commands mixin

Move Matrix /status scope output and /resume same-room guards from the
pre-refactor gateway/run.py into gateway/slash_commands.py so PR #18505
foundation behavior survives the upstream god-file decomposition.

Uses i18n keys for Matrix resume/status messages. Preserves upstream
session.py fixes (role_authorized, DM user_id isolation).

* docs(matrix): explain inbound dispatch via handle_sync loop

Document why Hermes uses an explicit sync loop with handle_sync() rather than
client.start(), aligning with upstream #7914 diagnostics while preserving
Hermes background maintenance tasks.

* fix(i18n): add Matrix resume/status keys to all locale catalogs

The Matrix /resume and /status slash-command keys added in the foundation
PR must exist in every supported locale file. tests/agent/test_i18n.py
asserts key and placeholder parity across catalogs.

Non-English locales use English strings as interim placeholders until
community translators can localize them.

* fix(matrix): restore gateway authz for allowed_users; honor config require_mention

Revert the early MATRIX_ALLOWED_USERS gate in _on_room_message so inbound
sender authorization stays in gateway authz like main. Parse require_mention
from config.extra (platforms.matrix / top-level matrix yaml) with env fallback,
matching thread_require_mention and fixing Forge when require_mention is set
only in profile config.yaml.

* fix(matrix): harden status scope and allowlisted DMs

* fix(matrix): use session store lookup for resume scope

tests/gateway/test_session.py

@@ -611,6 +611,30 @@ class TestSessionStoreSwitchSession:
         db.close()
 
 
+class TestSessionStoreLookupBySessionId:
+    @pytest.fixture()
+    def store(self, tmp_path):
+        config = GatewayConfig()
+        with patch("gateway.session.SessionStore._ensure_loaded"):
+            s = SessionStore(sessions_dir=tmp_path, config=config)
+        s._db = None
+        s._loaded = True
+        return s
+
+    def test_returns_active_entry_for_persisted_session_id(self, store):
+        source = SessionSource(
+            platform=Platform.MATRIX,
+            chat_id="!room:example.org",
+            chat_type="group",
+            user_id="@alice:example.org",
+        )
+        entry = store.get_or_create_session(source)
+
+        assert store.lookup_by_session_id(entry.session_id) is entry
+        assert store.lookup_by_session_id("missing") is None
+        assert store.lookup_by_session_id("") is None
+
+
 class TestWhatsAppSessionKeyConsistency:
     """Regression: WhatsApp session keys must collapse JID/LID aliases to a
     single stable identity for both DM chat_ids and group participant_ids."""