fix(skills): install Codex user skills to ~/.agents/skills - #14154

Fixes #14153

Description

gh skill install --agent codex --scope user still installed skills to the deprecated ~/.codex/skills directory. Codex now documents ~/.agents/skills as its user-level skill directory, and project-scoped installs already follow the matching .agents/skills convention.

This changes the Codex registry entry to use the shared .agents/skills directory at user scope and adds regression coverage for that path.

How did you test this change?

Given a temporary home directory and the repository's local code-review skill,
when I ran the built CLI with skill install . code-review --from-local --allow-hidden-dirs --agent codex --scope user,
then the command reported installation in ~/.agents\skills, the installed SKILL.md existed under that directory, and no skill was created under ~/.codex/skills.

Key points

This updates the canonical registry path only. It does not move existing files from ~/.codex/skills; users with an existing gh-managed installation there can reinstall the skill to place it in the current directory. gh skill list and gh skill update will use the new registry path as well.

Notes for reviewers

The functional change is the Codex UserDir entry in internal/skills/registry/registry.go, followed by the new user-scope case in TestInstallDir.

#13681 made the same directory correction for the Universal host and is the closest prior change.

Authorship and follow-up

Who wrote this:

• A human wrote it.
• An agent wrote it under close human direction.
• An agent wrote it independently, and no human has guided the implementation beyond the initial prompt.

Who answers review comments:

• @username will read and reply directly. Name the account.
• An agent will draft replies and @scarletkc will read them before they are posted.
• Nobody has explicitly committed to replying.