feat(runtime): add official monitor plugin

[GTC2333]

Summary

Stage 2 PR 5/9.

• add the official Monitor plugin package and bundled marketplace manifest
• provide bounded, event-driven background process monitoring with start_monitor, list_monitors, and stop_monitor
• wake the owning agent automatically when monitor output arrives; there is intentionally no wait_monitor
• keep Monitor disabled unless the corresponding marketplace capability is enabled
• preserve the Stage 1 task-delivery concurrency and follow-up behavior

Activation lifecycle

Agent tool lists are fixed when each agent is created. Enabling Monitor applies to new sessions and subsequently created agents. Already-running agents require a new session or a Runtime restart to receive Monitor tools.

Verification

• root typecheck
• 63 targeted runtime tests
• diff whitespace validation

Merge notes

Targets main directly and is independent of the remaining stage branches.

[GTC2333]

Fixed the failing task follow-up regression in f5a67f3. Monitor delivery had added streaming guards at both wake and delivery-loop entry, which also blocked durable task notifications from being injected into the active Principal turn. The guards now defer only Monitor-only delivery; queued task events still use the existing follow-up path. Validation: the previously failing regression passed 10/10 repeated runs, and the combined Monitor/task/server suite passed 68/68 with root typecheck.

[Fxde42]

Request changes：当前产品契约尚未统一。PR Summary 承诺显式 start/wait/stop，但实现实际是 start/list/stop 加输出到达时自动唤醒；同时，启用 Monitor 后已经创建的 agent 不会热加载新工具。建议保留事件驱动设计，不增加 wait_monitor，并同步修改 PR/spec 文案；同时在 UI 或文档明确已有 agent 需要新 session 或重启才能获得工具。若仍要求显式 wait，则需要实现 wait_monitor 及对应集成测试。

[GTC2333]

Addressed the product-contract review in 26a19af and the PR description. Monitor remains intentionally event-driven: start_monitor / list_monitors / stop_monitor, with automatic owner wake-up and no wait_monitor. Runtime documentation now states that existing agents keep their creation-time tool list and need a new session or Runtime restart. Validation: root typecheck and 63 targeted runtime tests.