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/commands/mac-security.md

@@ -0,0 +1,25 @@
+---
+allowed-tools: Bash(bash *audit_security.sh*)
+description: Security posture assessment — SIP, Gatekeeper, Firewall, FileVault, SSH, ports
+---
+
+## Your task
+
+Run a read-only security posture assessment.
+
+### Step 1 — Run the audit
+
+```bash
+bash $CLAUDE_PLUGIN_ROOT/skills/mac-optimizer/scripts/audit_security.sh
+```
+
+### Step 2 — Report
+
+Parse JSON output and present as a severity-ranked markdown table:
+
+| Severity | Finding | Recommended Action | Details |
+|---|---|---|---|
+
+### Step 3 — Guidance only
+
+The security module is **read-only**. Present findings and remediation guidance. Do NOT attempt to change security settings. Explain how the user can fix issues themselves via System Settings or Terminal commands.