feat: off the record mode (OTR)

Adds a third input mode toggle alongside Note and Agent. When active:
- Textarea gets a subtle purple tint with dashed border
- OTR button highlights purple
- Placeholder reads "Off the record — not logged or distilled…"
- off_record=True is sent to /chat; session_logger is skipped
- In-memory session context is preserved within the session

Switching to Note or Agent mode deactivates OTR, and vice versa.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/routers/chat.py

@@ -23,6 +23,7 @@ class ChatRequest(BaseModel):
     include_long: bool = True
     include_mid: bool = True
     include_short: bool = True
+    off_record: bool = False  # skip session log (in-memory context preserved)
     user: str = "scott"
     persona: str = "inara"
 
@@ -92,7 +93,8 @@ async def _stream_chat(req: ChatRequest):
             response_text, actual_backend = task.result()
             history.append({"role": "assistant", "content": response_text})
             save_session(session_id, history)
-            log_turn(session_id, req.message, response_text)
+            if not req.off_record:
+                log_turn(session_id, req.message, response_text)
 
             requested = req.model or settings.primary_backend
             payload = {