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

instructions/context-engineering.instructions.md

@@ -1,5 +1,6 @@
 ---
 description: 'Guidelines for structuring code and projects to maximize GitHub Copilot effectiveness through better context management'
+applyTo: '**'
 ---
 
 # Context Engineering
@@ -41,7 +42,3 @@ Principles for helping GitHub Copilot understand your codebase and provide bette
 - **Missing context**: Open the relevant files in tabs, or explicitly paste code snippets.
 - **Stale suggestions**: Copilot may not see recent changes. Re-open files or restart the session.
 - **Generic answers**: Be more specific. Add constraints, mention frameworks, reference existing code.
-
----
-
-*Assisted by [Claude](https://claude.ai)*