Unify provider identity lists around the first-class registry

[TheGreatAxios]

Summary

• FIRST_CLASS_PROVIDERS is the canonical static registry for provider identity; the Codex/xAI live-fetch fallbacks stay separate and a divergence test pins their agreement
• The ChatGPT-OAuth fallback default matches the OpenAI API-key path default, so both auth paths serving OpenAI resolve the same model
• The same test covers the Zen and quirks catalogs, which resolve through the registry entry

Verification

• bun run typecheck, bun run build, and bun run test pass
• bun run check passes: 7138 pass, 0 fail across 489 files

Operator note: fresh ChatGPT-OAuth default flips gpt-5.5 → gpt-5.4 to match the OpenAI API-key path (CL-5691).

Fixes CL-5691

[linear-code]

CL-5691

[TheGreatAxios]

Audit: Approve, no findings. The unification is sound: FIRST_CLASS_PROVIDERS is the canonical static registry, the Codex/xAI live-fetch fallbacks stay separate, and identity-divergence.test.ts pins their agreement, including the gpt-5.4 default alignment across both OpenAI-serving auth paths. Stored-default-guard evidence: applyPersistedOAuthDefaults (src/config/index.ts:113) folds any persisted settings.providers defaultModel into the projected registry entry, prepending it when unknown, so a stored default can neither silently revert nor dangle after this change. One-word nit applied and pushed as 1147b4a: this shim only renames -> this alias only renames in the xai constants comment, matching the Local alias header one line up.