Phase 4c: migrate session and registry messaging state to plans

[sandl99]

Summary

Phase 4c of #3896 migrates onboard-session and sandbox registry messaging state to the manifest-backed SandboxMessagingPlan.

Scope

• Persist onboard session state as messagingPlan instead of messagingChannels, messagingChannelConfig, and disabledChannels.
• Persist sandbox registry state as messaging.plan instead of top-level messagingChannels, messagingChannelConfig, and disabledChannels.
• Update onboard, rebuild, status, doctor, inventory, and channel lifecycle flows to derive channel config, active channels, and disabled channels from plans.
• Remove stale legacy conflict/backfill code and tests for the old messaging fields.
• Make this a clean cutover: no compatibility fallback or legacy backfill is required for messagingChannels, messagingChannelConfig, or disabledChannels.

Closing PR

Closed by #4945.

[wscurran]

✨
Related open PRs:

• #4945 refactor(messaging): persist session registry state as plans

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Related open issues:

• #3896 Refactor messaging integrations into a manifest-first planning architecture