feat(sign_in): 新增独立每日签到插件，与抽卡签到共用同一份签到数据

- 群聊命令「签到」(别名: 每日签到/打卡)，白名单群聊或管理员可用
- 复用 xapi /bot/gacha/sign-in + /bot/points/add，与抽卡/WPF 共用 bot_gacha_daily_sign_in 表
- 积分随机 1-100，流水沿用 gacha_sign/抽卡签到；先到先得，重复签到提示已签到
- 新增 10 个单元测试，ruff 通过；更新 PLUGINS.md / README.md

.trellis/tasks/08-16-standalone-sign-in-plugin/task.json

@@ -0,0 +1,26 @@
+{
+  "id": "standalone-sign-in-plugin",
+  "name": "standalone-sign-in-plugin",
+  "title": "独立每日签到插件（与抽卡签到共用数据）",
+  "description": "",
+  "status": "in_progress",
+  "dev_type": null,
+  "scope": null,
+  "package": null,
+  "priority": "P2",
+  "creator": "Mr.Xia",
+  "assignee": "Mr.Xia",
+  "createdAt": "2026-08-16",
+  "completedAt": null,
+  "branch": null,
+  "base_branch": "main",
+  "worktree_path": null,
+  "commit": null,
+  "pr_url": null,
+  "subtasks": [],
+  "children": [],
+  "parent": null,
+  "relatedFiles": [],
+  "notes": "",
+  "meta": {}
+}