[codex] Respect --brand lark in config init --new

[jnuyao]

Summary

• respect --brand lark in config init --new
• preserve the requested Lark brand in the saved config and JSON output
• surface the existing Lark endpoint retry message during app registration
• add a regression test for the --new --brand lark path

Root cause

config init --new hard-coded core.BrandFeishu when entering the create-app flow, so the requested brand never reached the setup URL generation path.

Testing

• go test ./cmd/config
• go test ./internal/auth ./internal/core
• go test ./...
  • this still fails in internal/registry on pre-existing tests:
    • TestColdStart_UsesEmbedded
    • TestCacheHit_WithinTTL

[CLAassistant]

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