- 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
