Phase 2b: persist manifest plans for channel lifecycle and rebuild

[sandl99]

Summary

Track phase 2b of the manifest-first messaging migration: make stored SandboxMessagingPlan data the durable source for channel lifecycle operations after enrollment.

Scope

• Persist the manifest plan in SandboxEntry.messaging.plan after channels add.
• Update stored manifest plans for channels stop, channels start, and channels remove without recompiling non-onboarding workflows.
• Stage the stored manifest plan during rebuild so the recreated sandbox can preserve manifest-driven messaging state.
• Keep legacy messagingChannels and disabledChannels behavior intact for sandboxes without a stored manifest plan.

Non-goals

• Do not remove legacy channel lifecycle code yet.
• Do not compile non-onboarding plans from legacy registry fields.
• Do not move policy/provider/render application fully to the applier in this phase.

Parent: #3896