feat: add richer worktree listing with wtk list

[nettee]

What to build

Add wtk list, a command that lists all worktrees managed/visible to Worktree Kit with a richer and more intuitive presentation than git worktree list.

The command should help users quickly understand what worktrees exist, where they live, what branch/commit they point to, and whether any worktree needs attention.

Acceptance criteria

• wtk list lists all relevant worktrees for the current repository.
• Output is more readable than raw git worktree list, with clear columns/sections for path, branch, commit, and status where applicable.
• The main worktree and linked worktrees are distinguishable.
• The command surfaces useful state such as missing/prunable/stale/dirty indicators when that information is available.
• The command has deterministic behavior and exits non-zero with a clear error when it cannot determine the repository/worktree state.
• Tests cover normal repositories, multiple linked worktrees, and at least one problematic/stale worktree case.

Blocked by

None - can start immediately