feat: OpenAI-compatible orchestrator + backend auto-routing

- openai_orchestrator.py — new ReAct tool loop engine for any
  OpenAI-compatible endpoint (OpenRouter, Open WebUI, Ollama, LiteLLM);
  model handles both tool loop and final response, no Claude handoff needed
- tools/__init__.py — auto-derive OpenAI JSON Schema from existing Gemini
  FunctionDeclarations so tool definitions have a single source of truth
- routers/orchestrator.py — route to openai_orchestrator when model registry
  "orchestrator" role resolves to a local_openai type host
- routers/chat.py — pass role to _backend_label(); fix fallback_used logic
  (only meaningful for explicit backend overrides, not auto-routing)
- static/app.js — add null/"auto" to backend cycle; fetch local model hint
  without overriding the auto default on page load
- model_registry.py — _normalize() back-fills host_type on old registry files
- requirements.txt — add openai>=1.0.0
- ARCH__BACKENDS.md — document OpenAI-compat backend and routing logic

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

cortex/static/style.css

@@ -565,6 +565,26 @@
         }
         .model-tag.fallback { color: #f59e0b; }
 
+        /* Retry button — shown in error message bubbles */
+        .retry-btn {
+            display: inline-block;
+            margin-top: 0.6rem;
+            margin-left: 0.15rem;
+            padding: 0.25rem 0.7rem;
+            font-size: 0.78rem;
+            font-family: inherit;
+            background: transparent;
+            color: var(--error-text);
+            border: 1px solid var(--error-border);
+            border-radius: 4px;
+            cursor: pointer;
+            transition: background 0.15s, color 0.15s;
+        }
+        .retry-btn:hover {
+            background: var(--error-border);
+            color: #fff;
+        }
+
         /* Note messages */
         .message.note-private {
             align-self: flex-end;