[codex] Remove Vercel OAuth login

[Che-Zhu]

Summary

• Remove Vercel OAuth sign-in/callback routes, Vercel auth client helpers, Vercel session refresh logic, and GitHub connect/disconnect flow.
• Collapse auth/session runtime to GitHub-only while preserving legacy accounts mapping for old Vercel users who had linked GitHub.
• Remove Vercel auth env/docs and @vercel/sdk while preserving non-auth Vercel preview, analytics, speed insights, AI Gateway, and legacy sandbox references.

Validation

• pnpm format
• pnpm format:check
• pnpm type-check
• pnpm lint (0 errors, existing warnings remain)
• pnpm exec tsx --test lib/auth/iframe-oauth.test.ts lib/auth/oauth-removal-regression.test.ts
• Static rg checks for Vercel auth/connect residues returned no results

Notes

• plan.md includes pre-deploy SQL audits for legacy Vercel users and duplicate GitHub account mappings.
• CODE_WIKI.md was left untracked and is not part of this PR.