`omp worktree clear` (without `--all`) removed every task-isolation dir
under the worktree base, including sandboxes owned by subagents running
right now, and the "no live task owns it" reason was asserted from the
mere presence of the `m` mount dir with no ownership check.
`ensureIsolation` now stamps each sandbox base dir with a pid-bearing
ownership marker before the backend materialises `m`, and the worktree
scanner classifies a sandbox as live while its owning process is alive,
so `clear` reclaims only crashed leftovers.
Fixes#6761
roboomp
eeca809193