feat: expand hello-world into a complete cross-compat plugin scaffold

Adds every cross-compatible component between Claude Code and Copilot CLI:
skills (shared), agents (per-tool .md vs .agent.md), hooks (per-tool
paths), and .mcp.json (shared). Both plugin.json manifests now carry all
shared identity fields (name, description, author, license, keywords)
with Copilot-specific component paths only in the root manifest.

Updates validate.sh sync check to verify shared identity fields rather
than demanding content equality, since component path declarations
legitimately diverge between tools.

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

plugins/hello-world/.mcp.json

@@ -0,0 +1,9 @@
+{
+  "mcpServers": {
+    "hello-world": {
+      "type": "stdio",
+      "command": "node",
+      "args": ["${CLAUDE_PLUGIN_ROOT}/bin/mcp-server.js"]
+    }
+  }
+}