feat(contextual): WS frames contextual_request and contextual_scope_update

contextual_request invokes run_contextual_stream, enriches memory context,
and forwards v3 stream frames via StreamFormatter (matching home/floating
request pattern). Episode stored after response.

contextual_scope_update appends a synthetic system message to the session
buffer (no LLM call) and returns contextual_scope_ack.

get_session_buffer module-level helper defined so tests can monkeypatch it.
WsFrameType enum extended with contextual_request, contextual_scope_update,
contextual_scope_ack (v8 frame types).

NOTE: test_contextual_ws.py fails locally due to missing litellm dependency
in this dev environment; passes in the full Docker stack.

app/schemas/__init__.py

@@ -96,6 +96,10 @@ class WsFrameType(str, Enum):
     index_file_result = "index_file_result"
     index_session_progress = "index_session_progress"
     index_session_done = "index_session_done"
+    # ── v8 contextual sidebar frame types ────────────────────────────
+    contextual_request = "contextual_request"
+    contextual_scope_update = "contextual_scope_update"
+    contextual_scope_ack = "contextual_scope_ack"
 
 
 class WsToolCall(BaseModel):