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

gateway/run.py

@@ -2221,6 +2221,13 @@ class GatewayRunner:
         # are system-generated and must skip user authorization.
         if getattr(event, "internal", False):
             pass
+        elif source.user_id is None:
+            # Messages with no user identity (Telegram service messages,
+            # channel forwards, anonymous admin actions) cannot be
+            # authorized — drop silently instead of triggering the pairing
+            # flow with a None user_id.
+            logger.debug("Ignoring message with no user_id from %s", source.platform.value)
+            return None
         elif not self._is_user_authorized(source):
             logger.warning("Unauthorized user: %s (%s) on %s", source.user_id, source.user_name, source.platform.value)
             # In DMs: offer pairing code. In groups: silently ignore.
@@ -6597,6 +6604,8 @@ class GatewayRunner:
             chat_id=context.source.chat_id,
             chat_name=context.source.chat_name or "",
             thread_id=str(context.source.thread_id) if context.source.thread_id else "",
+            user_id=str(context.source.user_id) if context.source.user_id else "",
+            user_name=str(context.source.user_name) if context.source.user_name else "",
         )
 
     def _clear_session_env(self, tokens: list) -> None:
@@ -6809,6 +6818,8 @@ class GatewayRunner:
         platform_name = watcher.get("platform", "")
         chat_id = watcher.get("chat_id", "")
         thread_id = watcher.get("thread_id", "")
+        user_id = watcher.get("user_id", "")
+        user_name = watcher.get("user_name", "")
         agent_notify = watcher.get("notify_on_complete", False)
         notify_mode = self._load_background_notifications_mode()
 
@@ -6864,6 +6875,8 @@ class GatewayRunner:
                                 platform=_platform_enum,
                                 chat_id=chat_id,
                                 thread_id=thread_id or None,
+                                user_id=user_id or None,
+                                user_name=user_name or None,
                             )
                             synth_event = MessageEvent(
                                 text=synth_text,