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%

.env.example

@@ -0,0 +1,18 @@
+# MarkBase環境變數配置範例
+
+# API配置（server.rs:192）
+# restore_tree功能使用的API key與URL
+RESTORE_API_KEY=muser_your_api_key_here
+RESTORE_API_URL=http://localhost:3002/api/v1/files
+
+#伺服器配置
+SERVER_PORT=11438
+DB_DIR=data/users
+
+#日誌配置（未來實作）
+LOG_LEVEL=info
+
+# Runner配置（Gitea Actions）
+# 註冊Runner時取得的Token（僅首次註冊需要）
+# GITEA_RUNNER_TOKEN=your_runner_token_here
+# GITEA_INSTANCE=https://gitea.momentry.ddns.net