Fix delegated write evidence for owned child sessions

[Trecek]

A successful write-required run could be demoted to zero_writes when its only edit came from an owned child. Admit successful owned-child writes only when they match an invocation-bound target whose content changed, while preserving separate root write counts.

Declare dry-walkthrough's plan_path as its delegated target, bind it from canonical recipe or standalone inputs, and preserve the digest baseline across resume. Thread one evidence scope through normal and recovery paths, and report delegated_write_count only in summary.json. Unowned, denied, unbound, unchanged, and out-of-scope writes remain insufficient; worktree temp-only exclusions remain enforced.

Fixes #5285. After merge, integrate this fix into #5255 before fresh commissioning.

Validation:

• pre-commit run --all-files passed.
• task test-local-gate: 47,860 passed, 614 skipped, 29 xfailed.
• Replayed the retained artifact from run 38960069882: a changed target succeeds with zero root writes and one delegated write; an unchanged target returns zero_writes and logs delegated_writes_not_admitted.