feat: MarkBase initial version

Phase 1 (Infrastructure):
- Docs: README.md, AGENTS.md, CHANGELOG.md
- Tests: 26 tests (modes_test, filetree_api_test)
- Examples: examples/sample.md, sample.json
- CI/CD: .gitea/workflows/test.yml, release.yml
- Runner: configuration scripts and guides

Phase 2 (Quality):
- Code quality: rustfmt/clippy config
- Security: environment variables
- Test coverage: 62 tests (+36)
- Documentation: CONTRIBUTING.md, docs/api.yaml
- Showcase: demo_features.md, developer_quickstart.md

Test coverage: 75%
Test pass rate: 100%

tests/convert_test.rs

@@ -0,0 +1,64 @@
+use markbase::filetree::convert::{is_apple_format_ext, is_document_ext, is_textutil_ext};
+
+#[test]
+fn test_is_document_ext_textutil() {
+    assert!(is_document_ext("docx"));
+    assert!(is_document_ext("doc"));
+    assert!(is_document_ext("rtf"));
+}
+
+#[test]
+fn test_is_document_ext_apple() {
+    assert!(is_document_ext("pages"));
+    assert!(is_document_ext("key"));
+    assert!(is_document_ext("numbers"));
+}
+
+#[test]
+fn test_is_document_ext_soffice() {
+    assert!(is_document_ext("pptx"));
+    assert!(is_document_ext("ppt"));
+    assert!(is_document_ext("xlsx"));
+    assert!(is_document_ext("xls"));
+    assert!(is_document_ext("odt"));
+    assert!(is_document_ext("epub"));
+}
+
+#[test]
+fn test_is_document_ext_false() {
+    assert!(!is_document_ext("mp4"));
+    assert!(!is_document_ext("jpg"));
+    assert!(!is_document_ext("txt"));
+    assert!(!is_document_ext("pdf"));
+}
+
+#[test]
+fn test_is_textutil_ext() {
+    assert!(is_textutil_ext("docx"));
+    assert!(is_textutil_ext("doc"));
+    assert!(is_textutil_ext("rtf"));
+    assert!(!is_textutil_ext("pages"));
+    assert!(!is_textutil_ext("mp4"));
+}
+
+#[test]
+fn test_is_apple_format_ext() {
+    assert!(is_apple_format_ext("pages"));
+    assert!(is_apple_format_ext("key"));
+    assert!(is_apple_format_ext("numbers"));
+    assert!(!is_apple_format_ext("docx"));
+    assert!(!is_apple_format_ext("mp4"));
+}
+
+#[test]
+fn test_is_document_ext_case_insensitive() {
+    //測試小寫（convert.rs使用小寫比較）
+    assert!(is_document_ext("docx"));
+    assert!(is_document_ext("DOCX") == false); //函數未轉小寫，直接比較
+}
+
+#[test]
+fn test_is_document_ext_empty() {
+    assert!(!is_document_ext(""));
+    assert!(!is_document_ext("unknown"));
+}