Add Codex compatibility discovery

[K-Mistele]

Summary

• discover Codex project and global instructions with override and local-file precedence
• discover ancestor and enabled plugin skills with canonical path and model-visible name deduplication
• model FileSystem.FileSystem as an ambient Effect requirement, supplied by the Node platform layer at runtime and replaceable in tests

Validation

• bun run typecheck
• bun run test
• bun run lint
• bun run format:check