feat(maestro): worktree on by default with new-or-existing branch picker (#61)

- toggle defaults on for any target the project scan reports (repo or
  worktree); only the free-text escape hatch pays for /api/gitroot
- branch mode control switches between creating a new branch (named
  after the session name, as before) and picking an existing one from
  GET /api/branches; branches held by another worktree stay visible but
  disabled, labelled with that worktree's path
- emits create_branch: false on the wire for existing-mode picks
- branch-list load follows the form's ready/destroyed discipline so a
  late response cannot corrupt a rebuilt draft

web/src/api.ts

@@ -1,6 +1,7 @@
 import * as routes from "./routes.js";
 import type {
   ApiError,
+  Branches,
   CreateSessionRequest,
   DaemonInfo,
   DirListing,
@@ -89,4 +90,11 @@ export async function fetchNames(count: number): Promise<NameBatch> {
   return jsonOrError(await fetch(url, { credentials: "same-origin" }));
 }
 
+/** One repository's local branches, with worktree occupancy (#55). Empty for
+ * a path outside any repo. Drives the spawn form's existing-branch picker. */
+export async function fetchBranches(path: string): Promise<Branches> {
+  const url = routes.withPathQuery(routes.BRANCHES, path);
+  return jsonOrError(await fetch(url, { credentials: "same-origin" }));
+}
+
 export { eventsWsUrl } from "./routes.js";