Add Grok compatibility discovery

[K-Mistele]

Summary

• load Grok-native and compatible project instructions, rules, skills, and plugin skills with deterministic precedence
• keep Codex-only roots isolated while respecting project ignores and reporting malformed plugin metadata nonfatally
• require ambient FileSystem.FileSystem through Effect and let runtime composition supply the Node implementation

Validation

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

Dependency

• stacked on Add Codex compatibility discovery #24, which adds the shared compatibility discovery foundation