fix(config): use Hermes config.yaml as single source of default model (#773)

Removes split-brain where WebUI Settings persisted default_model separately from Hermes runtime config.yaml. New POST /api/default-model endpoint writes to config.yaml. Existing saved values migrated on first load.

Fixes #761

Co-authored-by: aronprins <aronprins@users.noreply.github.com>

api/routes.py

@@ -47,6 +47,7 @@ from api.config import (
     CHAT_LOCK,
     load_settings,
     save_settings,
+    set_hermes_default_model,
 )
 from api.helpers import (
     require,
@@ -881,6 +882,14 @@ def handle_post(handler, parsed) -> bool:
         s = new_session(workspace=workspace, model=body.get("model"))
         return j(handler, {"session": s.compact() | {"messages": s.messages}})
 
+    if parsed.path == "/api/default-model":
+        try:
+            return j(handler, set_hermes_default_model(body.get("model")))
+        except ValueError as e:
+            return bad(handler, str(e))
+        except RuntimeError as e:
+            return bad(handler, str(e), 500)
+
     if parsed.path == "/api/sessions/cleanup":
         return _handle_sessions_cleanup(handler, body, zero_only=False)