Add changelog fragment and update Phase 2 status in plan doc

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/changelog.d/dagger-phase2-docs.feature.md

@@ -0,0 +1 @@
+Migrate docs build pipeline to Dagger (Phase 2): `dagger call build-docs --src=. --version=dev` now runs the full Quartz build locally, identically to CI. Adds `date-modified` frontmatter to all docs and a `docs-check-frontmatter` pre-commit hook.

docs/how-to/plans/adopt-dagger-ci.md

@@ -10,7 +10,7 @@ tags:
 
 # Plan: Adopt Dagger as CI/CD Build Engine
 
-> **Status:** Phase 1 implemented
+> **Status:** Phase 2 implemented
 
 ## Background
 
@@ -497,11 +497,11 @@ BuildKit caches aggressively, making repeated builds fast. Since the Forgejo run
 - [x] Existing `mise run container-tag-and-release` workflow still works end-to-end
 
 ### Phase 2 (Docs)
-- [ ] `dagger call build-docs --src=. --version=dev` produces valid tarball locally
-- [ ] Tarball contents match current Quartz build output
-- [ ] `dagger call release-docs` uploads to Forgejo packages successfully
-- [ ] Quartz container starts and serves docs from Forgejo packages URL
-- [ ] ArgoCD sync works from within Dagger
+- [x] `dagger call build-docs --src=. --version=dev` produces valid tarball locally
+- [x] Tarball contents match current Quartz build output
+- [ ] ~~`dagger call release-docs` uploads to Forgejo packages successfully~~ (deferred — artifact hosting stays on Forgejo Releases)
+- [ ] ~~Quartz container starts and serves docs from Forgejo packages URL~~ (deferred)
+- [ ] ~~ArgoCD sync works from within Dagger~~ (deferred)
 - [ ] Forgejo Actions workflow_dispatch completes full release cycle
 - [ ] CHANGELOG.md and fragment cleanup committed correctly