chore: remove activity.md from version control

Add to .gitignore to prevent development notes from being tracked.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: PastaPastaPasta

.gitignore

@@ -175,3 +175,4 @@ test/lint/.cppcheck/*
 # Editor and tooling
 .vscode/
 compile_commands.json
+activity.md