prune command reports false success when branch deletion fails for squash-merged PRs

[bjester]

Observed Behavior

When worktrees associated with squash-merged GitHub pull requests are pruned, local branch deletion fails because git branch -d rejects branches whose commit history does not form a direct ancestry with the target branch. Despite this failure, _do_prune catches the exception, logs a warning, and returns True. The command summary then reports full success (1 pruned, 0 failed), misleading the user into believing the branch was removed when it was actually retained.

Expected Behavior

1. Worktrees associated with merged pull requests (already cryptographically verified via GitHub PR headRefOid == local_head) should have their local branches deleted cleanly (or handle squash-merge semantics safely).
2. If branch deletion fails, _do_prune must reflect the failure accurately, incrementing failed_count and reporting the failure in the final summary.

Steps to Reproduce

1. In a repository, create and push a branch, open a PR, and squash-merge it on GitHub.
2. In the local repository, run treefort prune.
3. Confirm prompt (y).
4. Git worktree directory is removed, but git.delete_branch executes git branch -d -- <branch>, which exits with non-zero status:
   error: The branch '<branch>' is not fully merged.
5. _do_prune catches CommandError, logs warning Failed to delete branch '<branch>': ..., but still returns True.
6. Command reports: Pruning complete: 1 pruned, 0 failed. Local branch still exists in git branch.

Minimal Reproduction Environment / Conditions

• Version / Branch / Commit: release-v0.5.x@6d07c0c
• Component: treefort.commands.prune.PruneCommand
• Trigger: Any GitHub PR merged via "Squash and merge" (default GitHub merge method)

Errors, Logs & Telemetry

WARNING: Failed to delete branch 'feature-squashed': Command failed, with exit status 1: git branch -d -- feature-squashed
INFO: Successfully pruned 'feature-squashed'
INFO: Pruning complete: 1 pruned, 0 failed.

Deliverables & Testing Requirements

• Address branch deletion for squash-merged PRs (once local_head == pr.headRefOid confirms the branch was merged into the PR, determine whether force deletion -D or prompt is appropriate).
• Ensure failed_count increments when branch deletion fails, and avoid logging Successfully pruned when a core step of pruning fails.
• Regression Safety: Add a unit test in tests/commands/test_prune.py asserting that _do_prune returns False or reports failure when delete_branch raises CommandError.

Technical Pointers & Root Cause Clues

• Suspected Components: src/treefort/commands/prune.py:44-52, 119-132 (_do_prune and __call__), src/treefort/git.py:87-90 (delete_branch)
• Related Commits: bd7ed4c (Add new prune command for removing worktrees for merged PR branches)

Notes & Edge Cases

• Squash-merging is the predominant merge strategy on GitHub. Without addressing this, the majority of PRs pruned by users will leave stale branches behind.

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.