Add AGENTS.md + tighten contribution flow

- AGENTS.md: public contributor/agent guide (two-file rule, model-centric
  ids, one-entry-per-harness, bilingual notes, screenshot proof)
- PR template: require a generation proof screenshot; drop the manual
  update-registry step; add id-naming and no-README-edit checks
- CI: PRs only run validate-data; pushes to main auto-regenerate and commit
  the README registry (contents:write on that job, [skip ci], no loop)
- README/update-registry: reflect auto-synced registry; fix stale 'two cases'
  -> three (mythos-craft, pelican-cycling, skeleton-watch); link AGENTS.md

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/update-registry.ts

@@ -85,8 +85,8 @@ function table(lang: 'zh' | 'en'): string {
   const pending = groups.filter((group) => group.runs === 0).map((group) => escapeCell(group.label))
   const note =
     lang === 'zh'
-      ? '> 此表由 `bun scripts/update-registry.ts` 从 `models/*.json` 自动生成；CI 会检查它是否最新。'
-      : '> This table is generated from `models/*.json` by `bun scripts/update-registry.ts`; CI checks that it stays current.'
+      ? '> 此表由 `bun scripts/update-registry.ts` 从 `models/*.json` 生成；合并到 `main` 后由 CI 自动同步，贡献者无需手改。'
+      : '> This table is generated from `models/*.json` by `bun scripts/update-registry.ts`; CI re-syncs it automatically after merge — contributors need not hand-edit it.'
   const header =
     lang === 'zh'
       ? '| 模型 | 厂商 | 运行环境（Harness）× 思考配额 | 产出 |'