fix(tui): show external worktree session labels - #41147

What

Restore session-directory labels for sessions outside the canonical project directory, such as sessions from sibling Git worktrees.

The earlier label cleanup removed every footer in current-directory mode. That avoided repeating labels for sessions inside the main project, but it also hid the only distinction between sessions returned from sibling worktrees that share the same project ID and subpath.

This keeps labels hidden for the canonical project root and directories beneath it, while showing the directory basename for external worktrees—even when the TUI is currently running in that worktree. All-projects mode is unchanged.

Testing

• bun typecheck in packages/tui
• Main worktree active: main session unlabeled and external sibling labeled
• External worktree active: the active worktree session remains labeled, while the canonical main session is unlabeled
• Nested canonical-project directory active: canonical directories remain unlabeled and the matching external-worktree directory is labeled
• All-projects mode: every session continues to show its project label

All scenarios were exercised in the real TUI against one isolated server and seeded Git repository.

Requested by: @jlongster (James Long via Slack)