feat(plugin): conform root manifest to Agent Plugins 1.0.0

Add the $schema field the Agent Plugins spec (agent-plugins.org) requires,
plus author/homepage/repository/license/keywords, and correct the stale
version to 0.6.6. Our skills/<name>/SKILL.md layout already matches the
spec's skills convention, so this manifest is the only change needed.

Existing .claude-plugin and .codex-plugin manifests are untouched.

plugin.json

@@ -1,5 +1,11 @@
 {
+  "$schema": "https://agent-plugins.org/schemas/1.0.0/plugin.schema.json",
   "name": "agent-skills",
-  "version": "1.0.0",
-  "description": "Production-grade engineering skills for AI coding agents."
+  "version": "0.6.6",
+  "description": "Production-grade engineering skills for AI coding agents covering the full software development lifecycle from spec to ship.",
+  "author": { "name": "Addy Osmani", "url": "https://github.com/addyosmani" },
+  "homepage": "https://skills.addy.ie",
+  "repository": "https://github.com/addyosmani/agent-skills",
+  "license": "MIT",
+  "keywords": ["skills", "engineering", "spec", "tdd", "review", "ship"]
 }