Add .gitignore covering OS, editor, Go artifacts, secrets, and scan data

.gitignore

@@ -0,0 +1,33 @@
+# OS
+.DS_Store
+Thumbs.db
+
+# Editors
+*.swp
+*.swo
+*~
+*.bak
+.idea/
+.vscode/
+*.sublime-*
+
+# Node
+node_modules/
+
+# Environment / secrets
+.env
+.env.*
+*.pem
+*.key
+
+# Go build artifacts
+/sfdupes
+*.test
+*.out
+*.log
+
+# Local scan data
+files.dat
+
+# Agent worktrees
+.claude/worktrees/