Add reference/ to .gitignore

Cloned imessage-exporter source for API research. The reference/
directory contains cloned repos for reading, not for committing.

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

Co-Authored-By: Claude <[email protected]>

Author: ndbroadbent

.gitignore

@@ -38,3 +38,6 @@ src-tauri/icons/ios/
 package-lock.json
 yarn.lock
 pnpm-lock.yaml
+
+# Reference repos (cloned for reading, not committed)
+reference/