docs: streamline project documentation

[kaiiiichen]

Summary

• replace the bilingual, badge-heavy README with a concise English-only project overview
• add DEVELOPMENT.md as the single technical reference for setup, configuration, architecture, routes, APIs, validation, and deployment
• simplify contribution, agent, Claude, security, portfolio, environment, and pull request documentation
• add .codex/README.md with repository-local Codex guidance
• update the commit hook and agent policy so Codex commits use Co-authored-by: Codex <noreply@openai.com> and never add Claude
• align page descriptions with the current live site and current implementation

Impact

Documentation is shorter, easier to scan, and less likely to drift because detailed development guidance now has one source of truth. Codex collaboration and commit-credit rules are now explicit and enforced by the repository hook. No runtime behavior changes.

Test plan

• scanned tracked documentation for Chinese characters and stale bilingual anchors
• checked all local Markdown link targets
• validated the commit hook syntax and resulting commit trailers
• verified both PR commits credit Codex and neither credits Claude
• npm run lint
• npm run typecheck
• npm run test (23 tests)
• npm run build (36 routes/pages generated)
• reviewed the current live homepage, About, Projects, Music for Life, and Misc pages

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Project	Deployment	Actions	Updated (UTC)
kaichen.dev	Ready	Preview, Comment	Jul 26, 2026 1:46am