fix(gateway): propagate user identity through process watcher pipeline

Background process watchers (notify_on_complete, check_interval) created
synthetic SessionSource objects without user_id/user_name. While the
internal=True bypass (1d8d4f28) prevented false pairing for agent-
generated notifications, the missing identity caused:

- Garbage entries in pairing rate limiters (discord:None, telegram:None)
- 'User None' in approval messages and logs
- No user identity available for future code paths that need it

Additionally, platform messages arriving without from_user (Telegram
service messages, channel forwards, anonymous admin actions) could still
trigger false pairing because they are not internal events.

Fix:
1. Propagate user_id/user_name through the full watcher chain:
   session_context.py → gateway/run.py → terminal_tool.py →
   process_registry.py (including checkpoint persistence/recovery)

2. Add None user_id guard in _handle_message() — silently drop
   non-internal messages with no user identity instead of triggering
   the pairing flow.

Salvaged from PRs #7664 (kagura-agent, ContextVar approach),
#6540 (MestreY0d4-Uninter, tests), and #7709 (guang384, None guard).

Closes #6341, #6485, #7643
Relates to #6516, #7392

tests/tools/test_notify_on_complete.py

@@ -227,6 +227,8 @@ class TestCheckpointNotify:
             "session_key": "sk1",
             "watcher_platform": "telegram",
             "watcher_chat_id": "123",
+            "watcher_user_id": "u123",
+            "watcher_user_name": "alice",
             "watcher_thread_id": "42",
             "watcher_interval": 5,
             "notify_on_complete": True,
@@ -236,6 +238,8 @@ class TestCheckpointNotify:
             assert recovered == 1
             assert len(registry.pending_watchers) == 1
             assert registry.pending_watchers[0]["notify_on_complete"] is True
+            assert registry.pending_watchers[0]["user_id"] == "u123"
+            assert registry.pending_watchers[0]["user_name"] == "alice"
 
     def test_recover_defaults_false(self, registry, tmp_path):
         """Old checkpoint entries without the field default to False."""