Decouple avatar asset from app workspace

[NWYLZW]

Summary

• remove assets/avatar from the app root pnpm workspace so app frozen install no longer validates the avatar asset site's package.json
• document the new Avatar Pages boundary: the target repo installs/builds independently and aliases app-source for avatar source/CSS
• update GitHub Actions and package agent notes to keep future deploy changes on the independent path

Related

• Requires the target workflow change in Fix Avatar Pages build outside app workspace avatar#1 before the app trigger can deploy through the new path.

Verification

• pnpm install --frozen-lockfile
• pnpm -C packages/avatar test
• pnpm exec dprint check .github/workflows/deploy-avatar.yml pnpm-workspace.yaml .github/AGENTS.md .oo/rules/maintenance/README.md .oo/rules/release/avatar-github-pages.md packages/avatar/AGENTS.md packages/avatar/README.md

Experience Review

• 已判断是否需要沉淀经验
• 如需要，已运行 $post-task-experience-review
• reviewer PASS 后才进入 merge

[github-actions]

Experience Review

Before merge, make sure the PR body contains this completed checklist:

## Experience Review
- [x] 已判断是否需要沉淀经验
- [x] 如需要，已运行 `$post-task-experience-review`
- [x] reviewer PASS 后才进入 merge

Checklist to confirm:

• Decide whether this PR produced reusable project experience.
• If experience should be captured, run $post-task-experience-review.
• Merge only after the independent reviewer reports PASS.