📝 docs: rewrite AGENTS.md for JevTown

[nevamind-agent]

动机

AGENTS.md came in with the initial squash commit from remaining-time and was never updated. Most
of it didn't apply to this repo, and some of it was actively misleading:

• It named dev as the integration branch and PR base. JevTown has no dev, and an agent following
  that rule would branch from a different project's history.
• It pointed to experiment/tavern-agentic, the tavern scope rules, and remaining-time PR #68, none
  of which exist here.
• It froze docs/ and sent architecture docs to a separate docs repo, while this repo keeps its
  design docs in numbered docs/NN-*.md files.
• It told agents not to run tests or checks by default, and required a Codex/PI_MODEL trailer.

解决方案

Rewrite AGENTS.md in English so it matches this repo:

• Branches and PRs: main is the main branch. feat/agentic is the development branch for
  agentic features, and agentic PRs target it. Always pass --base explicitly.
• CI: lists the checks that run on PRs into main (Prettier, ESLint, tsc --noEmit, npm test,
  PR title validation) and the Prettier pre-commit hook, without a "don't test" rule.
• Docs: docs/ is open again, with the docs/NN-topic.md naming and (docs/13 §2) citations.
• Commits and PRs: keeps the <gitmoji> <type>: <subject> style. It notes that
  validate-pr-title.yml enforces it on PR titles only (gitmoji optional) and that commit messages
  aren't checked. Also keeps the PR template and the Co-authored-by trailer.

Not in this PR: .agents/skills/* and this PR template still describe remaining-time, and
feat/agentic doesn't have main's CI workflows.

类型

• ✨ feat: 添加新功能
• 🐛 fix: 修复 bug
• 📝 docs: 对文档进行修改
• ♻️ refactor: 代码重构（既不是新增功能，也不是修改 bug 的代码变动）
• ⚡ perf: 提高性能的代码修改
• 🧑‍💻 dx: 优化开发体验
• 🔨 workflow: 工作流变动
• 🏷️ types: 类型声明修改
• 🚧 wip: 工作正在进行中
• ✅ test: 测试用例添加及修改
• 🔨 build: 影响构建系统或外部依赖关系的更改
• 👷 ci: 更改 CI 配置文件和脚本
• ❓ chore: 其它不涉及源码以及测试的修改
• ⬆️ deps: 依赖项修改
• 🔖 release: 发布新版本

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