fix(codex): install skills into Codex skill root

[dommonkhouse]

Summary

• install Codex plugin skills into ~/.codex/skills instead of ~/.agents/skills
• update Codex plugin docs and uninstall guidance to match the Codex skill root
• keep memory-recall guidance clear about Hermes vs MemSearch runtime impact

Verification

• git diff --check origin/main..HEAD
• bash -n plugins/codex/scripts/install.sh
• scanned added lines for local/user-specific paths: no matches for /Users, dominic, monkhouse, claude-config, mcp-setup, or .agents/skills

[dommonkhouse]

Ready for maintainer review and merge. I verified this PR is non-draft, mergeable, and has no configured checks reporting failures.