fix(plugin): declare skills and agents paths in plugin.json

Claude Code's plugin loader requires explicit path declarations when
any path field (like `commands`) is already present. Without `skills`
and `agents` fields, auto-discovery is suppressed and all 21 skills
return "Unknown skill" when invoked via the Skill tool.

Adds:
  "skills": "./skills"
  "agents": "./agents"

This makes all 21 skills discoverable and invokable by name (e.g.
`code-review-and-quality`) and registers the 3 agent personas
(`code-reviewer`, `test-engineer`, `security-auditor`) as subagents.

Fixes #112

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

.claude-plugin/plugin.json

@@ -8,5 +8,7 @@
   "homepage": "https://github.com/addyosmani/agent-skills",
   "repository": "https://github.com/addyosmani/agent-skills",
   "license": "MIT",
-  "commands": "./.claude/commands"
+  "commands": "./.claude/commands",
+  "skills": "./skills",
+  "agents": "./agents"
 }