fix: restore Codex cook skill discovery

[Trecek]

Summary

• expose the projected Codex cook catalog from the generated CODEX_HOME/skills
  discovery root using relative, same-home symlinks
• preserve the existing profile skill when a profile/catalog name collision occurs
• retain the physical add-dir/skills catalog so headless Codex behavior is unchanged
• add regression coverage for discovery links and the temporary collision policy

Scope

This is a containment hotfix for the regression documented in #4357. It does
not select the long-term skill-delivery architecture, and #4357 should remain
open for remediation of discovery contracts, collision precedence, and a
real-Codex invocation canary.

Validation

• focused task test-check: 31 passed
• worktree task test-check: 30,655 passed, 2,572 skipped, 27 expected xfails
• pre-commit run --all-files: passed

Refs #4357