feat: replace Agent mode with independent Tools toggle

- Remove 'agent' from mode dropdown; Chat/Note/OTR remain
- Add ⚡ tools toggle button in input bar (persisted in localStorage)
  When on: routes to POST /orchestrate (Gemini tool loop); send btn → "Run"
  When off: routes to POST /chat (direct to active role); no change
- Role selector and tools toggle are now fully independent:
  active chat_role sent in orchestrate payload → used for final response
- orchestrator_engine.run() accepts response_role param; passes it to
  complete(role=...) instead of hardcoded model="claude"
- OrchestrateRequest gains chat_role field (default "chat")
- Migrate stored 'agent' mode/MRU entries to 'chat' on load

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

cortex/static/index.html

@@ -176,6 +176,8 @@
             <div id="mode-dropdown"></div>
             <!-- Note visibility sub-toggle — only shown when note mode is active -->
             <button id="note-vis-btn" title="Toggle note visibility (private / public)">prv</button>
+            <!-- Tools toggle — routes through the orchestrator tool loop when active -->
+            <button id="tools-toggle" title="Tools disabled — click to enable">⚡</button>
         </div>
         <textarea id="input" rows="1" placeholder="Message…" autofocus></textarea>
         <div id="send-col">