fix(worktree): recover empty cleanup shells

[dawNotPoi]

Summary

• recover cleanup when Git has already detached a worktree but left an empty directory shell behind
• remove only a proven-empty shell before pruning stale registration and deleting the task branch
• preserve fail-closed behavior for non-empty or unreadable paths while leaving normal worktree removal unchanged

Testing

• cargo test --no-default-features --features test-utils --lib worktree_ -- --nocapture (41 passed)
• cargo check --no-default-features --features test-utils --lib
• cargo clippy --no-default-features --features test-utils --lib --tests -- -D warnings
• git diff --check

Repository-wide cargo fmt --check still reports pre-existing formatting differences outside this change; the modified hunks were checked separately.

Closes #642