feat: add VS Code + GitHub Copilot agent skills support

- Add `.copilot-plugin/plugin.json` for auto-discovery in VS Code (mirrors `.cursor-plugin/` format)
- Add `.vscode/INSTALL.md` with two install options: auto-discovery and personal skills via symlink
- Update README.md and README.zh-CN.md with VS Code section and compatibility table row

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

.copilot-plugin/plugin.json

@@ -0,0 +1,15 @@
+{
+  "name": "understand-anything",
+  "displayName": "Understand Anything",
+  "description": "AI-powered codebase understanding — analyze, visualize, and explain any project",
+  "version": "2.0.0",
+  "author": {
+    "name": "Lum1104"
+  },
+  "homepage": "https://github.com/Lum1104/Understand-Anything",
+  "repository": "https://github.com/Lum1104/Understand-Anything",
+  "license": "MIT",
+  "keywords": ["codebase-analysis", "knowledge-graph", "architecture", "onboarding", "dashboard"],
+  "skills": "./understand-anything-plugin/skills/",
+  "agents": "./understand-anything-plugin/agents/"
+}