feat: import per-model metadata and protect catalog fields

[dongyu23]

PR2 follow-up to #1943 and metadata portion of closed #1837. This branch is based on the PR1 branch so its metadata commit is ready to review on top of #1943. It adds models.json import, protected Codex-managed fields, slug synchronization, and external catalog conflict validation with rollback. After #1943 merges, the PR1 commits will disappear from this diff and this PR will contain only the metadata changes. Validation: npm test (81 passed), npm run check, cargo relay_config and launcher tests passed, git diff --check.

[dongyu23]

@BigPizzaV3 This is the PR2 follow-up requested in closed #1837. It contains the metadata portion only, on top of PR1 #1943:

• models.json import with same-slug matching
• protected Codex-managed fields
• model deletion and metadata persistence fixes
• pure API provider identity preservation

The branch is based on the latest main and has passed local tests plus fork CI validation so far.

[BigPizzaV3]

此 PR 依赖 #1943，不能在基础 PR 未同步和验证前单独合并。请把 base 指向更新后的 #1943（或在 #1943 合并后 rebase main），解决当前冲突并补 metadata 导入不会覆盖用户显式窗口/压缩阈值的测试，再触发 CI。