Don't silently swallow "failed to delete" Worktree error

I marked a worktree session as done. When I later checked the output for logs I saw `2026-07-24 10:33:22.819 [warning] [agentHostGitService] > git worktree remove --force c:\Code\vscode-vsce.worktrees\oidc-support-for-secretless-publishing failed; full stderr:
error: failed to delete 'C:/Code/vscode-vsce.worktrees/oidc-support-for-secretless-publishing': Permission denied`

I think we should probably show this error to the user? Or maybe it should appear in the sessions management UI @sandy081 is looking into