feat(agent-manager): non-blocking worktree setup with unified busy state (#532)

Show new worktrees in the sidebar immediately after creation with a
spinner icon, allowing users to switch to other worktrees while the
setup script runs instead of being blocked by a full-screen overlay.

- Add worktreeId to worktreeSetup messages so the webview can track
  which worktree each setup event belongs to
- Push state to sidebar immediately after git worktree creation so the
  entry appears before the setup script runs
- Replace separate deletingWorktrees/settingUpWorktrees signals with a
  unified busyWorktrees map (WorktreeBusyState with reason field) for
  consistent loading behavior across setup and deletion
- Scope the setup overlay to only show when the setting-up worktree is
  selected; switching away hides it, switching back restores it
- Hide the delete button on busy worktrees instead of showing a second
  spinner
- Ensure failed session creation cleans up stale sidebar entries via
  pushState() after removeWorktree()