refactor(todo): 去除 MySQL 依赖，改为 JSON 本地存储

- 数据存储: ~/.local/share/todo/tasks.json 纯字符串数组
- 操作简化: 仅保留 add/list/done，无状态/优先级/归档
- 新增 scripts/validate.py 校验脚本 (格式/结构/去重)
- 新增 schema/tasks.schema.json
- 删除 references/setup.md (MySQL 配置指南)

Co-Authored-By: Claude Code <noreply@anthropic.com>

todo/schema/tasks.schema.json

@@ -0,0 +1,16 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "type": "object",
+  "properties": {
+    "tasks": {
+      "type": "array",
+      "items": {
+        "type": "string",
+        "minLength": 1
+      },
+      "uniqueItems": true
+    }
+  },
+  "required": ["tasks"],
+  "additionalProperties": false
+}