Add a way to delete or manage the linked worktree for a session

### Feature summary

Add a way to delete or manage the linked worktree for a session

### What problem are you trying to solve?

Sessions create linked git worktrees, but there does not seem to be a convenient way to manage or remove the underlying worktree from the session workflow.

If a session is no longer needed, especially after abandoned, failed, or experimental runs, the linked worktree can remain on disk and has to be cleaned up manually outside the app. That makes it easy to accumulate stale worktrees and harder to understand which worktree belongs to which session.

### Proposed solution

Please add session-level worktree management. For example:

- a "Delete linked worktree" action on a session, or
- a clear option when deleting a session to also delete the underlying linked worktree.

If fully deleting it is not always safe, surfacing the linked worktree path and cleanup status on the session would still make manual management much easier.

### Workflow impact

This would make session cleanup much more predictable for users who create many sessions or use sessions for short-lived experiments. It would reduce stale worktrees, reclaim disk space more easily, and make session lifecycle management feel complete inside the app instead of requiring separate git or manual cleanup steps.

### Installation context

GitHub App sessions using linked git worktrees in local repositories

### Additional context

This seems related to the broader worktree and session experience, but is distinct from importing existing worktrees (#28) and from cleanup bugs where session deletion leaves git state behind (#42). This request is specifically about giving users an explicit, convenient way to manage or delete the linked worktree associated with a session.