feat(work): add ISO timestamps + auto-bump on staged work-doc changes

- New scripts/work/bump-updated-timestamps.mjs stamps the `updated:`
  frontmatter field to the current ISO 8601 UTC timestamp on every
  staged docs/work/**/*.md file. Idempotent; adds the field after
  `created:` if missing.
- .husky/pre-commit invokes the bump script as step 2 (before
  rebuild-state) so _state.json sees the fresh timestamp.
- Backfill all existing work docs (4 PRDs + 3 epics + 21 stories):
    * created: promoted from \`YYYY-MM-DD\` -> ISO timestamp using
      git log --diff-filter=A on each file (first-commit date for
      stories that had no \`created:\` line, midnight UTC for PRDs
      and epics that had date-only created).
    * updated: added from \`git log -1 --format=%aI\` on each file
      (last-commit timestamp); will be re-stamped to "now" by the
      pre-commit hook on this commit.

Stories that had no \`created:\` line now get one.

docs/work/2026-05-14-library-evaluation-policy/06-sandcastle-reviewer-prompt/_story.md

@@ -7,6 +7,8 @@ status: done
 feature: tooling
 depends-on: [02-pre-commit-check-script]
 blocks: []
+created: 2026-05-14T06:52:02+02:00
+updated: 2026-05-14T19:10:35.370Z
 ---
 
 ## Goal