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%

.gitignore

@@ -0,0 +1,31 @@
+# Rust
+/target
+Cargo.lock
+
+#測試暫存檔
+/data/users/test_*.sqlite
+
+# Runner配置
+/.runner
+
+#環境變數
+/.env
+.env.local
+
+#日誌檔案
+*.log
+*.tmp
+
+# macOS
+.DS_Store
+.AppleDouble
+.LSOverride
+
+# IDE
+.vscode/
+.idea/
+*.swp
+*.swo
+
+#緩存
+/data/cache/*.tmp