Fix review feedback: mode→agent frontmatter, add applyTo, generate plugin

- Fix prompt frontmatter: mode: 'agent' → agent: 'agent' (repo convention)
- Add applyTo: '**' to instructions file
- Remove trailing Claude credit from instructions
- Generate plugin via npm run plugin:migrate
- Rebase onto latest upstream/main

plugins/context-engineering/.github/plugin/plugin.json

@@ -0,0 +1,10 @@
+{
+  "name": "context-engineering",
+  "description": "Tools and techniques for maximizing GitHub Copilot effectiveness through better context management. Includes guidelines for structuring code, an agent for planning multi-file changes, and prompts for context-aware development.",
+  "version": "1.0.0",
+  "author": {
+    "name": "Awesome Copilot Community"
+  },
+  "repository": "https://github.com/github/awesome-copilot",
+  "license": "MIT"
+}