fix: exclude slash-command/setup messages from LLM context (#2634) (#2640)

Slash-command replies and the echoed /setup command are persisted to session
history so they render in the transcript, but they are UI chatter the user
never meant as conversation. They were sent to the model on the next turn,
which then commented on '/setup ...' and exposed transient values (e.g. the
Copilot device user_code) to the LLM.

- get_context_messages() (the LLM-API view) now skips messages tagged
  metadata.source == 'slash'. Display/history-load paths use raw history and
  are unaffected.
- slashCommands.js tags the echoed user command with source:'slash' too (the
  assistant replies already carried it); the user line was the one untagged
  path that still reached context.

Fixes #2634.

core/models.py

@@ -76,8 +76,20 @@ class Session:
             _session_manager._persist_message(self.id, message)
 
     def get_context_messages(self) -> List[Dict[str, Any]]:
-        """Get messages in format for LLM API."""
-        return [msg.to_dict() for msg in self.history]
+        """Get messages in format for LLM API.
+
+        Slash-command / setup replies are persisted to history so they render
+        in the transcript, but they are UI chatter (e.g. ``/setup ...`` and its
+        status lines) the user never meant as conversation. They carry
+        ``metadata.source == "slash"``; exclude them here so they never reach
+        the model. Display/history-load paths use the raw ``history`` and are
+        unaffected.
+        """
+        return [
+            msg.to_dict()
+            for msg in self.history
+            if (msg.metadata or {}).get("source") != "slash"
+        ]
 
     def get(self, key: str, default=None):
         """Dict-like access for compatibility."""