Add Codex plugin support

[schmiegelow]

Summary

• add Codex plugin manifests and a repository marketplace index for all three plugin shapes: xp-clean-code, pr-validation, and spec-driven
• expose all four spec-driven skills to Codex while preserving the Claude command adapters
• keep skill metadata portable across Claude Code, Cursor, and Codex, including the merged five-analysis PR validation workflow
• discover plugin directories dynamically during cross-host validation so future plugins cannot omit a host manifest or Codex marketplace entry unnoticed
• document Codex installation and explicit skill invocation for every plugin

Validation

• python3 scripts/validate_manifests.py
• validate_plugin.py passed for all three plugins
• quick_validate.py passed for all six skills
• python3 -m py_compile scripts/validate_manifests.py plugins/spec-driven/skills/spec/references/spec-sync-guard.py
• git diff origin/main --check