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/app.js

@@ -80,11 +80,35 @@
 
         agentModeBtnEl.addEventListener('click', () => {
             agentMode = !agentMode;
+            if (agentMode) { otr_mode = false; update_otr_ui(); }
             localStorage.setItem('agentMode', agentMode);
             updateAgentModeUI();
             inputEl.focus();
         });
 
+        // ── Off the record mode ──────────────────────────────────────
+        let otr_mode = false;
+        const otr_btn_el = document.getElementById('otr-btn');
+
+        function update_otr_ui() {
+            otr_btn_el.classList.toggle('active', otr_mode);
+            inputEl.classList.toggle('otr-mode', otr_mode);
+            updateInputPlaceholder();
+        }
+
+        otr_btn_el.addEventListener('click', () => {
+            otr_mode = !otr_mode;
+            if (otr_mode) {
+                agentMode = false;
+                noteMode = false;
+                localStorage.setItem('agentMode', false);
+                updateAgentModeUI();
+                updateInputMode();
+            }
+            update_otr_ui();
+            inputEl.focus();
+        });
+
         // ── Note mode ────────────────────────────────────────────────
         let noteMode   = false;
         let notePublic = false;
@@ -124,6 +148,8 @@
                 inputEl.placeholder = ctrlEnterMode
                     ? `Task for ${personaLabel}… (Gemini tool loop — Ctrl+Enter to run)`
                     : `Task for ${personaLabel}… (Gemini tool loop)`;
+            } else if (otr_mode) {
+                inputEl.placeholder = `Off the record — not logged or distilled…`;
             } else {
                 inputEl.placeholder = ctrlEnterMode
                     ? `Message ${personaLabel}… (Ctrl+Enter to send)`
@@ -133,6 +159,7 @@
 
         noteBtnEl.addEventListener('click', () => {
             noteMode = !noteMode;
+            if (noteMode) { otr_mode = false; update_otr_ui(); }
             updateInputMode();
             inputEl.focus();
         });
@@ -690,6 +717,7 @@
                         include_long: memLong,
                         include_mid: memMid,
                         include_short: memShort,
+                        off_record: otr_mode,
                         user: CORTEX_USER,
                         persona: CORTEX_PERSONA,
                     }),