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/2026-05-13-binder-wrap-helper/01-wire-use-case-helper/_story.md

@@ -7,6 +7,8 @@ status: done
 feature: core-shared
 depends-on: []
 blocks: [02-migrate-feature-binders, 03-update-generator-templates]
+created: 2026-05-13T19:17:10+02:00
+updated: 2026-05-14T19:10:35.370Z
 ---
 
 ## Goal

docs/work/2026-05-13-binder-wrap-helper/02-migrate-feature-binders/_story.md

@@ -7,6 +7,8 @@ status: done
 feature: auth, blog, media, marketing-pages, navigation
 depends-on: [01-wire-use-case-helper]
 blocks: []
+created: 2026-05-13T19:17:10+02:00
+updated: 2026-05-14T19:10:35.370Z
 ---
 
 ## Goal

docs/work/2026-05-13-binder-wrap-helper/03-update-generator-templates/_story.md

@@ -7,6 +7,8 @@ status: done
 feature: core-shared
 depends-on: [01-wire-use-case-helper]
 blocks: []
+created: 2026-05-13T19:17:10+02:00
+updated: 2026-05-14T19:10:35.370Z
 ---
 
 ## Goal

docs/work/2026-05-13-binder-wrap-helper/_epic.md

@@ -5,7 +5,8 @@ title: Collapse binder duplication via wireUseCase helper
 type: epic
 status: done
 features: [core-shared, auth, blog, media, marketing-pages, navigation]
-created: 2026-05-13
+created: 2026-05-13T00:00:00Z
+updated: 2026-05-14T19:10:35.370Z
 ---
 
 ## Goal