fix(cron): preserve reminder delivery target from session context

[YspCoder]

Motivation

• 修复 remind 工具创建定时任务时未携带会话目标（channel/chatID）的问题，导致 deliver=true 的任务触发时无法走外发投递链路。

Description

• 在 RemindTool 中新增 defaultChannel 和 defaultChatID 字段并实现 SetContext(channel, chatID) 来保存当前会话上下文。
• 在 RemindTool.Execute 中将 AddJob 的 channel/to 参数由空值替换为 t.defaultChannel / t.defaultChatID，确保持久化的任务包含投递目标。
• 在 AgentLoop 的工具上下文更新位置注册 remind 工具的 SetContext 调用，保证每条消息处理时刷新提醒工具的会话上下文。
• 新增回归测试 pkg/tools/remind_test.go 校验在设置上下文后创建的提醒任务其 Payload.Deliver、Payload.Channel 与 Payload.To 被正确持久化。

Testing

• 已运行格式化命令 gofmt -w pkg/tools/remind.go pkg/tools/remind_test.go pkg/agent/loop.go，格式化成功。
• 运行单元测试 go test ./pkg/tools ./pkg/agent ./pkg/cron -count=1，pkg/tools 和 pkg/agent 测试通过，pkg/cron 无测试文件。
• 新增回归测试 pkg/tools/remind_test.go 已执行并通过，验证提醒任务保存了正确的投递目标。

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[chatgpt-codex-connector]

Avoid shared mutable delivery context in RemindTool

This commit makes RemindTool store defaultChannel/defaultChatID as global mutable fields, but the same tool instance is shared across all sessions and AgentLoop processes sessions concurrently (see buildSessionShards/processMessage in pkg/agent/loop.go). When two chats are handled at the same time, one request can call SetContext between another request's context set and Execute, so AddJob persists the wrong Payload.Channel/Payload.To and reminders may be delivered to the wrong user; this is also an unsynchronized read/write data race.

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