fix: credit owned child writes to fresh declared outputs

[Trecek]

Fixes prepare-pr runs being reported as zero_writes after an owned child successfully writes the generated prep artifact.

Add delegated_write_output and opt prepare-pr in for prep_path. Bind a completed pre-dispatch baseline under the default skill output directory, then admit an owned successful write only when its canonical path matches the final declared artifact, is fresh and in scope, and remains a readable regular file. Keep child credit in delegated_write_count, preserving root counts and the existing delegated-input rules.

Validation:

• pre-commit run --all-files passed.
• The configured test_check gate (task test-local-gate) passed: 42,960 passed, 613 skipped, 29 xfailed.
• Three focused mutation checks detected the intended regressions: missing output declaration, missing freshness check, and missing scan-error handling.

Closes #5289.

After this merges to develop, integrate the fix into #5255 before fresh commissioning.