feat: finalize auth UX updates

[iam-brain]

Summary

• ports auth UX to ANSI-based interactive menus and removes legacy codex tool registrations
• hardens provider model default loading/normalization and updates modern config/docs examples
• refreshes tests and docs to align with new auth/menu and model config behavior
• preserves only legacy plugin-owned codex command cleanup while keeping user-defined codex-prefixed commands

Verification

• npm run typecheck
• npx vitest run test/plugin-config-hook.test.ts test/auth-menu.test.ts test/auth-menu-flow.test.ts test/auth-menu-runner.test.ts
• HOME="/tmp/opencode-audit-home.HAfOiI" npm test
• npm run build