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

prompts/context-map.prompt.md

@@ -1,5 +1,5 @@
 ---
-mode: 'agent'
+agent: 'agent'
 tools: ['codebase']
 description: 'Generate a map of all files relevant to a task before making changes'
 ---

prompts/refactor-plan.prompt.md

@@ -1,5 +1,5 @@
 ---
-mode: 'agent'
+agent: 'agent'
 tools: ['codebase', 'terminalCommand']
 description: 'Plan a multi-file refactor with proper sequencing and rollback steps'
 ---

prompts/what-context-needed.prompt.md

@@ -1,5 +1,5 @@
 ---
-mode: 'agent'
+agent: 'agent'
 tools: ['codebase']
 description: 'Ask Copilot what files it needs to see before answering a question'
 ---