fix(skills): stop mirroring skills into Codex and remove old mirror links

[lorenzozanee]

What

The global skills mirror no longer creates a Codex-specific copy: Codex reads the universal ~/.agents/skills store, so a second copy made every skill show up twice.

Why

Codex discovers the user-level universal skills store, so creating another Codex mirror causes duplicate skill discovery.

How

Add Codex to the universal-store reader capability set and skip it before directory creation or target replacement. Symlinks an earlier mirror left in the Codex skills dir (they resolve to the same Claude-store skill) are removed, but only when the universal store holds that skill and the Codex dir passes the same alias check every other destructive path runs; real directories and Windows copies carry no ownership proof, so they stay. Add isolated default and custom CODEX_HOME regression coverage.

Size

Small on purpose: one gate in the skills mirror plus a cleanup of the links it used to create, with tests. Extending the skip to the other agents that read the universal store is a separate follow-up.

Test plan

• Unit tests added/updated
• Manual testing performed (not applicable; isolated filesystem tests cover the behavior)
• Documentation updated (not applicable)

Focused tests, formatting, lint, and diff checks pass. Package typecheck remains unverified because unrelated generated modules are absent in the local worktree.

Fixes #3995