Rename AGENTS.md to CLAUDE.md, add root .gitignore

- Rename project instructions file to Claude Code standard name
- Add .gitignore to exclude IDE files and frontend-poc/

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

.gitignore

@@ -0,0 +1,12 @@
+# IDE
+.idea/
+.vscode/
+*.swp
+*.swo
+
+# OS
+.DS_Store
+Thumbs.db
+
+# Experimental/POC directories
+frontend-poc/

CLAUDE.md

@@ -1,4 +1,4 @@
-# AGENTS.md - Mantimon TCG
+# Mantimon TCG
 
 Guidelines for agentic coding agents working on this codebase.