Hosted 'serve' sessions can now surface host-side notices to the cloud session view

What

• In the headless-cloud attach/create path, when a session is serving tools, a new dedup store is created, gated by hostedServeDialogsGateOn (which falls back to checking gate tengu_violin_chinrest). When present, it collects notices via a new onNotice callback and exposes them through cloudSession()'s new notices field and a new onCloudSessionChanged hook.
• The cloud_session init-frame schema used by the headless stream-json client gained a notices array describing device-serving events the user should be shown: MCP consent/reconsent/drop, hook run status, bash-from-home, and mute/unmute of served tools.
• The notice store is a bounded map holding only the latest notice per key, with change subscription, so duplicate notices for the same event are deduplicated instead of firing ad hoc.

Why

This lets a cloud session view show the user what's happening on the host machine serving tools to it, such as a hook running or an MCP consent request, instead of those events being invisible or repeated, while the feature stays behind a gate until it's ready.