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/orchestrator_engine.py

@@ -58,6 +58,7 @@ async def run(
     respond_with_claude: bool = True,
     gemini_api_key: str | None = None,
     model_name: str | None = None,
+    response_role: str = "chat",
 ) -> OrchestratorResult:
     """
     Run the full orchestration loop for a task.
@@ -176,7 +177,7 @@ async def run(
         response_text, backend = await complete(
             system_prompt=system_prompt,
             messages=messages,
-            model="claude",
+            role=response_role,
         )
     else:
         # Cron/background tasks: return Gemini's summary directly, no Claude call