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/static/index.html

@@ -131,6 +131,7 @@
             <!-- Note mode controls -->
             <button id="note-type-btn">private</button>
             <button id="note-btn">Note</button>
+            <button id="otr-btn" title="Off the record — not logged or distilled into memory">OTR</button>
             <button id="agent-mode-btn" title="Agent mode — Gemini tool loop + Claude response">Agent</button>
             <button id="send">Send</button>
             <button id="stop">Stop</button>