refactor: convert mac-optimizer from raw skill to proper plugin

Restructured 92-mac-optimizer from a CLAUDE.md-based skill into a full
Claude Code plugin with .claude-plugin/plugin.json, 6 slash commands
(/mac-doctor, /mac-packages, /mac-environment, /mac-security,
/mac-cleanup, /mac-resources), and auto-trigger SKILL.md.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

custom-skills/92-mac-optimizer/skills/mac-optimizer/references/packages.md

@@ -0,0 +1,52 @@
+# Package Manager Reference
+
+## Homebrew
+
+### Update workflow
+```bash
+brew update                    # Update Homebrew itself and formulae list
+brew outdated                  # List outdated formulae
+brew outdated --cask           # List outdated casks
+brew upgrade                   # Upgrade all outdated formulae
+brew upgrade <name>            # Upgrade specific formula
+brew upgrade --cask            # Upgrade all casks
+brew cleanup --prune=0         # Remove all cached downloads
+brew autoremove                # Remove orphaned dependencies
+brew doctor                    # Diagnose issues
+```
+
+### Safety notes
+- `brew upgrade` is generally safe but can break projects pinned to specific versions
+- Always check `brew doctor` warnings — some are cosmetic, some indicate real issues
+- `brew autoremove` only removes packages not depended on by anything
+
+## npm / nvm
+
+### Update workflow
+```bash
+nvm ls                         # List installed Node versions
+nvm install --lts              # Install latest LTS
+npm outdated -g                # Check global packages
+npm update -g                  # Update all global packages
+npm cache clean --force        # Clear npm cache
+```
+
+### Safety notes
+- Global npm packages are version-independent from project dependencies
+- Updating global packages won't affect project-level node_modules
+
+## Python / pyenv
+
+### Update workflow
+```bash
+pyenv versions                 # List installed versions
+pyenv install --list           # Available versions
+pip list --outdated            # Outdated packages in current env
+pip install --upgrade <pkg>    # Upgrade specific package
+pip cache purge                # Clear pip cache
+```
+
+### Safety notes
+- Only audit pyenv-managed environments (not system Python)
+- pip upgrades can break dependency chains — suggest `--dry-run` first
+- Virtual environments in project directories are not scanned (out of scope v1)