fix: deploy Codex skills to the official user path

[N4M3Z]

fix: deploy Codex skills to the official user path

Summary

Rune deploys Codex skills under <target>/.codex/skills.

Current OpenAI Docs define $HOME/.agents/skills as the Codex user skill location.

The old target can create duplicate skills with different content.

Reproduction

1. Run rune install --source <deck> --target "$HOME" --provider codex.
2. Read the target layout in rune install --help.
3. Compare $HOME/.codex/skills with $HOME/.agents/skills.
4. Restart Codex and inspect the available skill paths.

Actual result

Rune writes the skill to $HOME/.codex/skills.

Codex can then list that copy beside an existing $HOME/.agents/skills copy.

Codex can select the stale copy because both descriptions match.

Expected result

Rune writes Codex user skills to $HOME/.agents/skills.

Rune keeps other Codex files in their supported locations.

Rune migrates its managed legacy skill files without deleting user-owned files.

Evidence

rune install --help currently reports:

codex → <DIR>/.codex

One local installation contained these two copies:

$HOME/.codex/skills/SimplifiedTechnicalEnglish/SKILL.md
$HOME/.agents/skills/simplified-technical-english/SKILL.md

The copies had different SHA-256 digests.

The active Codex session selected the stale .codex copy.

OpenAI Docs:

https://developers.openai.com/codex/skills#where-to-save-skills

Acceptance criteria

• The Codex provider deploys skills to <target>/.agents/skills.
• Existing managed .codex/skills files move to a recoverable quarantine.
• User-owned files remain unchanged.
• The deployment manifest records the new path.
• A repeated install does not create duplicate skills.
• Tests cover installation, migration, drift, and pruning.
• The CLI help lists the correct skill target.