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%

.rustfmt.toml

@@ -0,0 +1,11 @@
+# Rust格式化配置（stable版）
+
+edition = "2021"
+max_width = 100
+tab_spaces = 4
+
+#使用field_init_shorthand
+use_field_init_shorthand = true
+
+#使用try-contract宏
+use_try_shorthand = true