feat(core): port GitHub Copilot OAuth

[opencode-agent]

Summary

• port GitHub.com and Enterprise Copilot device OAuth to the V2 integration registry
• add credential-aware Copilot request headers for chat, responses, and messages endpoints
• port defensive remote model syncing while preserving local metadata and advertised endpoint selection
• cover OAuth registration, request headers, malformed/stale model handling, variants, and zero billing batch sizes

Closes #36335

Validation

• bun test test/github-copilot/models.test.ts test/plugin/provider-github-copilot.test.ts (12 passed)
• narrowed Core typecheck covering all changed source and tests
• scoped oxlint (no errors in changed source files)