chore: block agent-local files from git (.claude/ CLAUDE.md AGENTS.md etc)

Removes stale webui-mvp AGENTS.md and expands .gitignore to block all agent-local context files from being committed. Fixes .claude/* → .claude/ (directory block).

.gitignore

@@ -16,7 +16,13 @@ archive/
 .env
 .env.*
 !.env.example
-.claude/*
+.claude/
+CLAUDE.md
+AGENTS.md
+.cursorrules
+.windsurfrules
+.aider*
+copilot-instructions.md
 
 # Generated screenshots and transient artifacts
 screenshot-*.png