feat: server-side override pause/resume for builtin adapter types

Replace the client-side-only override store with a real server-side
toggle. When a developer pauses the external override, the server swaps
ALL adapter behavior back to the builtin — execute handler, model listing,
config schema, detection — not just the UI parser.

Server changes:
- registry.ts: builtinFallbacks map + pausedOverrides set + setOverridePaused()
- routes/adapters.ts: PATCH /api/adapters/:type/override endpoint + overridePaused in list

UI changes:
- adapters.ts: setOverridePaused API method + overridePaused on AdapterInfo
- AdapterManager: overrideMutation calls server, instant feedback via invalidate()
- use-disabled-adapters.ts: reads adapter.overridePaused from server response

Removed:
- disabled-overrides-store.ts: no longer needed (server is the source of truth)

Note: already-running agent sessions keep the adapter they started with.
Only new sessions use the swapped adapter.

ui/src/api/adapters.ts

@@ -19,6 +19,8 @@ export interface AdapterInfo {
   isLocalPath?: boolean;
   /** True when an external plugin has replaced a built-in adapter of the same type. */
   overriddenBuiltin?: boolean;
+  /** True when the external override for a builtin type is currently paused. */
+  overridePaused?: boolean;
 }
 
 export interface AdapterInstallResult {
@@ -43,6 +45,10 @@ export const adaptersApi = {
   setDisabled: (type: string, disabled: boolean) =>
     api.patch<{ type: string; disabled: boolean; changed: boolean }>(`/adapters/${type}`, { disabled }),
 
+  /** Pause or resume an external override of a builtin type. */
+  setOverridePaused: (type: string, paused: boolean) =>
+    api.patch<{ type: string; paused: boolean; changed: boolean }>(`/adapters/${type}/override`, { paused }),
+
   /** Reload an external adapter (bust server + client caches). */
   reload: (type: string) =>
     api.post<{ type: string; version?: string; reloaded: boolean }>(`/adapters/${type}/reload`, {}),