feat: unified model registry with role-based routing

Introduces model_registry.py as the single source of truth for all LLM
backend configuration. Replaces scattered backend settings across user_settings,
config distill_backend_*, and the UI toggle.

model_registry.py:
- Per-user home/{user}/model_registry.json with version, hosts, models, roles
- Models have: type (local_openai|claude_cli|gemini_cli|gemini_api), label,
  model_name, host_id, context_k (tokens), tags (capability labels)
- Roles map to priority chains: primary, backup_1..backup_4
- Built-in IDs (claude_cli, gemini_cli, gemini_api) always resolvable
- Auto-migrates existing local_llm.json on first access
- CRUD: save_host, remove_host, save_model, remove_model, set_role
- get_model_for_role(): registry → .env default → hardcoded fallback

config.py:
- role_chat/orchestrator/distill/coder/research .env defaults
- defined_roles: comma-separated standard role list (extensible)
- get_defined_roles() and get_role_default() helper methods

llm_client.complete():
- New role= parameter (default "chat") for registry-based routing
- model= still accepted for explicit UI toggle override
- _claude() and _local() accept model_cfg dict instead of raw string
- _local() uses pre-resolved config from registry

memory_distiller.py:
- distill_mid/long now use role="distill" (no more distill_backend_* .env vars needed)

cron_runner.py:
- brief jobs use role="chat"

routers/chat.py + auth.py:
- Use model_registry instead of user_settings for local model info

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

cortex/routers/auth.py

@@ -72,21 +72,33 @@ def _gemini_status() -> dict:
         return {"ok": False, "error": str(e), "warning": True, "authenticated": False}
 
 
-async def _local_status() -> dict:
-    if not settings.local_api_url:
+async def _local_status(username: str = "scott") -> dict:
+    """Check reachability of the user's configured local model host."""
+    import model_registry
+    cfg = model_registry.get_best_local_model(username)
+    if not cfg:
+        return {"configured": False}
+    api_url = cfg.get("api_url", "")
+    if not api_url:
         return {"configured": False}
     try:
         import httpx
-        url = settings.local_api_url.rstrip("/") + "/api/models"
+        url = api_url.rstrip("/") + "/api/models"
         headers = {}
-        if settings.local_api_key:
-            headers["Authorization"] = f"Bearer {settings.local_api_key}"
+        api_key = cfg.get("api_key", "")
+        if api_key:
+            headers["Authorization"] = f"Bearer {api_key}"
         async with httpx.AsyncClient(timeout=5) as client:
             resp = await client.get(url, headers=headers)
         reachable = resp.status_code < 400
-        return {"configured": True, "reachable": reachable, "model": settings.local_model}
+        return {
+            "configured": True,
+            "reachable": reachable,
+            "model": cfg.get("model_name", ""),
+            "label": cfg.get("label", ""),
+        }
     except Exception as e:
-        return {"configured": True, "reachable": False, "error": str(e), "model": settings.local_model}
+        return {"configured": True, "reachable": False, "error": str(e), "model": cfg.get("model_name", "")}
 
 
 @router.get("/status")