When a user cancels a plan after execute, the git worktree branch and
directory created during execute are not cleaned up, causing resource
leaks (dangling worktrees accumulate over time).
Add GitWorktreeSandbox.cleanup_stale() classmethod in the
infrastructure layer with idempotent error handling.
_cleanup_sandbox_for_plan() in the CLI layer resolves the plan's
linked git-checkout resource and delegates to cleanup_stale().
Called after service.cancel_plan() in the cancel CLI handler.
ISSUES CLOSED: #9230
