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%

examples/sample.json

@@ -0,0 +1,42 @@
+{
+  "title": "MarkBase範例 JSON",
+  "version": "1.0.0",
+  "features": [
+    {
+      "name": "Markdown渲染",
+      "status": "active",
+      "description": "支援完整 Markdown語法"
+    },
+    {
+      "name": "檔案樹管理",
+      "status": "active",
+      "description": "SQLite持久化檔案樹"
+    },
+    {
+      "name": "REST API",
+      "status": "active",
+      "description": "18+ RESTful路由"
+    },
+    {
+      "name": "音訊控制",
+      "status": "active",
+      "description": "macOS音訊裝置管理"
+    }
+  ],
+  "config": {
+    "port": 11438,
+    "db_dir": "data/users",
+    "cache_dir": "data/cache"
+  },
+  "display_modes": [
+    "tree",
+    "list",
+    "grid_sm",
+    "grid_lg"
+  ],
+  "metadata": {
+    "author": "MarkBase Team",
+    "created_at": "2026-05-16",
+    "updated_at": "2026-05-16"
+  }
+}