chore: update origin to cea6588bb3

[lacolaco]

Summary

Sync angular-ja with upstream angular/angular @ cea6588bb3 (cut for v21.2.15).

Includes upstream releases v21.2.11 ～ v21.2.15.

Migration breakdown

• Markdown 移行: 25 commits (per-(class, directory))
  • class-a: src/content/guide/templates
  • class-b: ai / guide / guide/components / guide/forms / guide/http / guide/signals / guide/testing / introduction / reference / tools/cli / tutorials/first-app/intro / tutorials/learn-angular/steps/5-control-flow-for
  • class-c: best-practices / guide / guide/animations / guide/components / guide/di / guide/http / guide/signals / guide/templates / guide/testing
  • class-d (additive defer): guide/forms/signals/models.md, guide/templates/{defer,variables}.md
  • roadmap: reference/roadmap.md (URL/属性ミラーのみ、JA翻訳保持)
• 新規未翻訳: fix: migrate untranslated files 2 commits — guide/forms/signals/field-metadata.md ほか
• ソース移行: 3 commits (src/app/features/update x2, src/app/routing/navigation-entries x1)
• lint fix: 1 commit (forms/signals/models.md の prh Signal Forms → シグナルフォーム)

Test plan

• pnpm test — 全 patch 適用OK
• pnpm lint — clean
• verify-migration.ts — 全 17 ディレクトリ pass
• verify-codeblock-sync.ts main — 39 changed files clean