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

[sandl99]

Summary

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

This migration is intentionally not backward-compatible: after this phase, messagingPlan in onboard sessions and messaging.plan in the sandbox registry are the only supported persisted messaging state.

Scope

• Persist onboard session messaging state only as messagingPlan; stop reading or writing messagingChannels, messagingChannelConfig, and disabledChannels in session state.
• Persist sandbox registry messaging state only as messaging.plan; stop reading or writing top-level messagingChannels, messagingChannelConfig, and disabledChannels in registry state.
• Treat plans as the source of truth for configured channels, active channels, disabled channels, and per-channel config values across onboard, sandbox build, policy, credentials, status, doctor, inventory, rebuild, and channel lifecycle flows.
• Remove legacy compatibility and backfill paths that derive plans, conflict state, channel config, or disabled-channel state from messagingChannels, messagingChannelConfig, or disabledChannels.
• Update channel add, remove, start, and stop mutations to read, mutate, and persist the plan object directly.
• Update unit, integration, and e2e fixtures to assert plan-backed session and registry state, and remove legacy-field compatibility expectations.

Non-goals

• No migration or backfill from legacy flat messaging fields.
• No compatibility shim for pre-plan onboard-session or sandbox-registry entries.

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