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>

.gitignore

@@ -3,3 +3,7 @@
 data
 config.yaml
 *.sql
+
+# Build artifacts
+gocv
+output/