chore: add .cgcignore to exclude dependencies and build artifacts

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

.cgcignore

@@ -0,0 +1,26 @@
+# Dependencies
+backend/vendor/
+frontend/node_modules/
+sabredav/vendor/
+
+# Build artifacts
+backend/bin/
+backend/init-scripts/
+frontend/build/
+frontend/.svelte-kit/
+
+# Test binaries
+backend/*.test
+
+# IDE
+.idea/
+.vscode/
+
+# OS & Config
+.git/
+.venv/
+bin/
+nginx/
+obsidian/
+# External repos
+dorf/