Add native Codex skills and workflow support

[arturodz]

Codex users can now install all 44 design skills and 18 command workflows from the same canonical sources as Claude Code and Gemini CLI. Workflows become native Codex skills, so $design-conversation runs the existing conversation-design workflow with the user's request as input.

• Generate and commit codex-skills/ through the existing build script, retaining knowledge skill contents and workflow bodies.
• Add a dependency-free Python installer for all collections or selected collections. It links into ~/.agents/skills (or --dest), checks conflicts before installation, and supports repeat installs without overwriting existing skills.
• Document setup and invocation, and extend the pre-commit hook and CI drift checks to include Codex and newly generated files.

Validation: all 62 generated skills passed the Codex skill validator; three regression tests passed covering canonical content preservation, deterministic rebuilds, stale skill removal, installation, repeat installation, conflict handling, and invalid metadata. git diff --check and shell syntax validation passed. Existing Claude and Gemini generated output is unchanged. Live model execution was not tested.