prune command fails to remove worktrees with slashes in their path

[bjester]

Observed Behavior

When worktrees are created within subdirectories or with paths containing slashes (e.g. feature/login), PruneCommand._iter_worktrees extracts Path(wt["path"]).name, truncating the path to only its basename (login). When _do_prune subsequently invokes git.remove_worktree("login"), git fails because no worktree named login exists in the project root.

Expected Behavior

PruneCommand should resolve the relative path of the worktree directory relative to project_dir so that nested worktrees (e.g. feature/login) can be accurately located and removed by git worktree remove.

Steps to Reproduce

1. Check out a worktree containing a slash in the branch/worktree name:
   treefort checkout feature/my-feature (creates directory <project_dir>/feature/my-feature).
2. Merge the corresponding PR on GitHub.
3. Run treefort prune --yes.
4. _iter_worktrees yields worktree_name = "my-feature" instead of "feature/my-feature".
5. git worktree remove -- my-feature fails with exit code 128: fatal: 'my-feature' is not a working tree.

Minimal Reproduction Environment / Conditions

• Version / Branch / Commit: release-v0.5.x@6d07c0c
• Component: treefort.commands.prune.PruneCommand
• Trigger: Any branch name containing slashes (e.g., feature/branch, bugfix/issue)

Errors, Logs & Telemetry

ERROR: Failed to prune 'my-feature': Command failed, with exit status 128: git worktree remove -- my-feature
fatal: 'my-feature' is not a working tree

Code location in src/treefort/commands/prune.py:72:

worktree_name = Path(wt["path"]).name  # Truncates 'feature/my-feature' to 'my-feature'

Deliverables & Testing Requirements

• Preserve relative worktree subpaths by computing Path(wt["path"]).resolve().relative_to(project_dir.resolve()).as_posix() rather than taking .name.
• Regression Safety: Add a unit test in tests/commands/test_prune.py verifying that a worktree at subpath/feature is correctly identified and pruned with its full relative path.

Technical Pointers & Root Cause Clues

• Suspected Components: src/treefort/commands/prune.py:72, 118
• Related Commits: bd7ed4c (Add new prune command for removing worktrees for merged PR branches)

Notes & Edge Cases

• Branches containing slashes (feature/*, fix/*) are standard git conventions and are explicitly supported by open_worktree in GitCLI.

Metadata

Severity / Priority: P0 (Blocker)
Area: bug, prune, git

AI Usage

Drafted with Gemini following the DEEP framework based on code review findings against release-v0.5.x. Reproduction conditions and code references were verified in the codebase.