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%

src/filetree/modes/list.rs

@@ -0,0 +1,87 @@
+use async_trait::async_trait;
+use serde_json::{json, Value};
+
+use crate::filetree::mode::{DisplayMode, FilterOption, SortOption};
+use crate::filetree::FileTree;
+
+pub struct ListMode;
+
+#[async_trait]
+impl DisplayMode for ListMode {
+    fn name(&self) -> &'static str {
+        "list"
+    }
+
+    fn render(&self, tree: &FileTree) -> Value {
+        let nodes: Vec<Value> = tree
+            .nodes
+            .iter()
+            .map(|n| {
+                json!({
+                    "node_id": n.node_id,
+                    "label": n.label,
+                    "aliases": n.aliases,
+                    "file_uuid": n.file_uuid,
+                    "sha256": n.sha256,
+                    "parent_id": n.parent_id,
+                    "node_type": n.node_type.as_str(),
+                    "icon": n.icon,
+                    "color": n.color,
+                    "bg_color": n.bg_color,
+                    "file_size": n.file_size,
+                    "registered_at": n.registered_at,
+                    "sort_order": n.sort_order,
+                })
+            })
+            .collect();
+
+        json!({
+            "mode": "list",
+            "user_id": tree.user_id,
+            "columns": ["icon", "label", "node_type", "updated_at"],
+            "nodes": nodes,
+        })
+    }
+
+    fn sort_options(&self) -> Vec<SortOption> {
+        vec![
+            SortOption {
+                key: "name_asc".into(),
+                label: "Name A-Z".into(),
+            },
+            SortOption {
+                key: "name_desc".into(),
+                label: "Name Z-A".into(),
+            },
+            SortOption {
+                key: "date_desc".into(),
+                label: "Newest First".into(),
+            },
+            SortOption {
+                key: "date_asc".into(),
+                label: "Oldest First".into(),
+            },
+            SortOption {
+                key: "type".into(),
+                label: "By Type".into(),
+            },
+        ]
+    }
+
+    fn filter_options(&self) -> Vec<FilterOption> {
+        vec![
+            FilterOption {
+                key: "all".into(),
+                label: "All".into(),
+            },
+            FilterOption {
+                key: "folder".into(),
+                label: "Folders".into(),
+            },
+            FilterOption {
+                key: "file".into(),
+                label: "Files".into(),
+            },
+        ]
+    }
+}