feat(buffer): ContextualBufferProxy + append_system_message

_SessionBuffer.append_system_message(user_id, session_id, text) injects a
synthetic SystemMessage into the named session slot (creating it if absent).

ContextualBufferProxy closes over user_id + session_id so call sites need
only call proxy.append_system_message(text).

get_session_buffer(user_id, session_id, channel) in device_ws returns a
ContextualBufferProxy, keeping the test-patchable function signature intact.

app/core/agent_session_buffer.py

@@ -54,6 +54,43 @@ class _SessionBuffer:
         with self._lock:
             self._store.pop((user_id, session_id), None)
 
+    def append_system_message(self, user_id: str, session_id: str, text: str) -> None:
+        """Append a synthetic system message to the buffer for the given session.
+
+        Creates the session slot if it does not yet exist.  Used by the
+        contextual_scope_update handler to inject navigation events without
+        making an LLM call.
+        """
+        from langchain_core.messages import SystemMessage  # noqa: PLC0415
+
+        key = (user_id, session_id)
+        with self._lock:
+            entry = self._store.get(key)
+            if entry is None:
+                msgs: list[BaseMessage] = [SystemMessage(content=text)]
+            else:
+                _, existing = entry
+                msgs = list(existing) + [SystemMessage(content=text)]
+            capped = msgs[-MAX_MESSAGES_PER_SESSION:]
+            self._store[key] = (time.monotonic(), capped)
+
+
+class ContextualBufferProxy:
+    """Thin wrapper around _SessionBuffer that closes over user_id + session_id.
+
+    Returned by get_session_buffer() so callers can call
+    ``proxy.append_system_message(text)`` without threading user_id/session_id
+    through every call site.
+    """
+
+    def __init__(self, buf: "_SessionBuffer", user_id: str, session_id: str) -> None:
+        self._buf = buf
+        self._user_id = user_id
+        self._session_id = session_id
+
+    def append_system_message(self, text: str) -> None:
+        self._buf.append_system_message(self._user_id, self._session_id, text)
+
 
 # Module-level singleton — same pattern as _pending_states in api/app/api/routes/auth.py
 session_buffer = _SessionBuffer()