chore: standardize .gitignore with AI file exclusions

Add standard patterns to exclude AI-generated markdown files:
- .claude/ directory
- CLAUDE.md and CLAUDE.MD files
- Top-level .md files with opt-in whitelist for documentation

Author: coenttb

.gitignore

@@ -31,10 +31,20 @@ Thumbs.db
 # Log files
 *.log
 
-# AI
-.claude
+# AI assistant files
+.claude/
+CLAUDE.md
 CLAUDE.MD
 
+# Documentation (opt-in for top-level .md files only)
+/*.md
+!README.md
+!LICENSE.md
+!CHANGELOG.md
+!CONTRIBUTING.md
+!CODE_OF_CONDUCT.md
+!SECURITY.md
+
 # Temporary files
 *.tmp
 *.temp