feat(ext/messages): add create_vault, attach_vault, get_vault_status (Plan C Phase 3 prep)

Adds the request shapes + response interfaces. POPUP_ONLY_TYPES set grows
by three. SW handlers in service-worker/vault.ts land in the next tasks.

The new union members would make popup-only.ts's exhaustive handle() switch
non-total (TS2366), so a default case is added returning an explicit
"unhandled popup message" error. create_vault/attach_vault get real cases
in Tasks 3.2-3.3; get_vault_status in Dev-C's Phase 6.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

extension/src/service-worker/router/popup-only.ts

@@ -627,6 +627,12 @@ export async function handle(
         return { ok: false, error: (e as Error).message };
       }
     }
+
+    // create_vault / attach_vault land in Phase 3 Tasks 3.2-3.3; get_vault_status
+    // in Phase 6 (Dev-C). Until each case lands, an unhandled popup message
+    // returns an explicit error rather than falling through with no return.
+    default:
+      return { ok: false, error: `unhandled popup message: ${(msg as { type: string }).type}` };
   }
 }