This fixes archive cleanup for agent threads so Zed only removes
worktrees it explicitly created, rather than treating every linked
worktree under the configured managed directory as safe to delete. When
Zed creates a worktree, it now records it in the local database along
with the creation time of the worktree's git metadata directory
(`.git/worktrees/<name>/`). Archive planning requires that record, and
right before deleting anything, Zed re-stats the directory and compares
creation times: if the worktree was removed and recreated outside Zed
(or the time can't be read at all), deletion is skipped and the stale
record is dropped. Every failure mode fails safe by leaving the
directory untouched.

For remote (SSH) projects, the stat runs on the remote host via a new
`GitWorktreeCreatedAt` request. Worktrees created by a different Zed
install (another release channel, or another machine connecting to the
same host) have no record in the local database and are therefore never
auto-archived, which is the intended conservative behavior.

Closes https://github.com/zed-industries/zed/issues/57155

Closes AI-352

Release Notes:

- Fixed archiving an agent thread incorrectly deleting manually-created
git worktrees.