Implement CLI mode and Markdown parsing

- Add CLI mode distinction (gocv vs gocv serve)
- Add goldmark dependency for Markdown parsing
- Create content reading logic from ./content directory
- Implement Markdown to HTML conversion
- Add sample content/index.md for testing
- Update .gitignore for build artifacts

Co-Authored-By: Claude (glm-5) <noreply@anthropic.com>

go.mod

@@ -6,3 +6,5 @@ require (
 	github.com/gorilla/mux v1.8.1
 	gopkg.in/yaml.v2 v2.4.0
 )
+
+require github.com/yuin/goldmark v1.7.16 // indirect