Make Codex integration skill-first

[Kirchberg]

Summary:

• Stop installing the deprecated Codex custom prompt at ~/.codex/prompts/sleep.md.
• Move any legacy ~/.codex/prompts/sleep.md from previous installs aside with a .skillopt-legacy*.bak suffix.
• Make the Codex skill the primary entrypoint by moving the launch commands, workflow steps, and safety rules into plugins/codex/skills/skillopt-sleep/SKILL.md.
• Update Codex docs, install output, and load-test notes to describe the skill-first flow instead of /sleep.

Why:

• Codex custom prompts are deprecated; reusable workflows should live in skills.
• The previous Codex integration split the launch flow between a custom prompt and a smaller skill. This makes the skill self-contained and removes the stale slash-command entrypoint.

Verification:

• bash -n plugins/codex/install.sh
• HOME=$(mktemp -d) bash plugins/codex/install.sh, then verified ~/.agents/skills/skillopt-sleep/SKILL.md exists and ~/.codex/prompts/sleep.md does not
• Created a legacy ~/.codex/prompts/sleep.md in a temp HOME, ran install.sh, and verified it moved to sleep.md.skillopt-legacy*.bak
• /Users/kirchberg/src/SkillOpt/.venv/bin/python -m pytest tests

This PR addresses #45 by making the Codex integration skill-first

I kept this separate from #48 so the Codex UX fix can be reviewed independently from the Codex Desktop transcript harvester