feat(plugins): add nteract Codex and Claude plugin configs

[rgbkrk]

Summary

• add a repo-local Codex plugin marketplace entry and plugins/nteract manifest/assets
• add a matching Claude plugin under .claude/plugins/nteract and register it in the local Claude marketplace/settings
• wire both plugin variants to the repo-native cargo run -q -p mcpb-runt entrypoint instead of checking in a bundled server artifact

Verification

• jq empty on the new/updated JSON files
• cargo xtask lint --fix

Notes

• the Codex plugin manifest still carries TODO placeholders for privacy policy and terms URLs because the current nteract metadata does not define them