Rename stderr-test-hook to user-message-hook for production

Changes:
- Renamed src/hooks/stderr-test-hook.ts to user-message-hook.ts
- Updated user-message-hook with production-ready messaging
- Updated scripts/build-hooks.js to build user-message-hook
- Updated plugin/hooks/hooks.json to reference user-message-hook.js
- Cleaned up old stderr-test-hook.js files
- Built and deployed user-message-hook.js to plugin directory

This hook displays context information to users via stderr, which is
currently the only way to show messages in Claude Code UI. It runs in
parallel with context-hook during SessionStart.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

scripts/build-hooks.js

@@ -18,7 +18,7 @@ const HOOKS = [
   { name: 'save-hook', source: 'src/hooks/save-hook.ts' },
   { name: 'summary-hook', source: 'src/hooks/summary-hook.ts' },
   { name: 'cleanup-hook', source: 'src/hooks/cleanup-hook.ts' },
-  { name: 'stderr-test-hook', source: 'src/hooks/stderr-test-hook.ts' }
+  { name: 'user-message-hook', source: 'src/hooks/user-message-hook.ts' }
 ];
 
 const WORKER_SERVICE = {