Add root .gitignore (#3)

- Ignore Claude Code worktrees
- Ignore common temporary files
- Ignore IDE/editor files
- Ignore build artifacts
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

+# Temporary files
+*.swp
+*.swo
+*~
+.DS_Store
+
+# Editor/IDE
+.vscode/*
+!.vscode/extensions.json
+!.vscode/settings.json
+.idea/
+
+# Build artifacts
+*.o
+*.obj
+*.exe
+*.out
+*.app
+
+# Claude Code worktrees
+.claude/worktrees/