[codex] Add PolyForm Perimeter license

[ovdmar]

Summary

• Add the PolyForm Perimeter License 1.0.1 text with a required notice pointing back to the original Citadel repository.
• Document Citadel as source-available in the README and clarify that commercial use is allowed when it does not provide a competing Citadel product or service.
• Add root package metadata pointing consumers to the repository LICENSE file.

Validation

• git diff --check
• pnpm exec biome check package.json