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/session_context.py

@@ -46,12 +46,16 @@ _SESSION_PLATFORM: ContextVar[str] = ContextVar("HERMES_SESSION_PLATFORM", defau
 _SESSION_CHAT_ID: ContextVar[str] = ContextVar("HERMES_SESSION_CHAT_ID", default="")
 _SESSION_CHAT_NAME: ContextVar[str] = ContextVar("HERMES_SESSION_CHAT_NAME", default="")
 _SESSION_THREAD_ID: ContextVar[str] = ContextVar("HERMES_SESSION_THREAD_ID", default="")
+_SESSION_USER_ID: ContextVar[str] = ContextVar("HERMES_SESSION_USER_ID", default="")
+_SESSION_USER_NAME: ContextVar[str] = ContextVar("HERMES_SESSION_USER_NAME", default="")
 
 _VAR_MAP = {
     "HERMES_SESSION_PLATFORM": _SESSION_PLATFORM,
     "HERMES_SESSION_CHAT_ID": _SESSION_CHAT_ID,
     "HERMES_SESSION_CHAT_NAME": _SESSION_CHAT_NAME,
     "HERMES_SESSION_THREAD_ID": _SESSION_THREAD_ID,
+    "HERMES_SESSION_USER_ID": _SESSION_USER_ID,
+    "HERMES_SESSION_USER_NAME": _SESSION_USER_NAME,
 }
 
 
@@ -60,6 +64,8 @@ def set_session_vars(
     chat_id: str = "",
     chat_name: str = "",
     thread_id: str = "",
+    user_id: str = "",
+    user_name: str = "",
 ) -> list:
     """Set all session context variables and return reset tokens.
 
@@ -74,6 +80,8 @@ def set_session_vars(
         _SESSION_CHAT_ID.set(chat_id),
         _SESSION_CHAT_NAME.set(chat_name),
         _SESSION_THREAD_ID.set(thread_id),
+        _SESSION_USER_ID.set(user_id),
+        _SESSION_USER_NAME.set(user_name),
     ]
     return tokens
 
@@ -87,6 +95,8 @@ def clear_session_vars(tokens: list) -> None:
         _SESSION_CHAT_ID,
         _SESSION_CHAT_NAME,
         _SESSION_THREAD_ID,
+        _SESSION_USER_ID,
+        _SESSION_USER_NAME,
     ]
     for var, token in zip(vars_in_order, tokens):
         var.reset(token)