feat(cloud): negotiate idempotent steer delivery

[tatoalo]

Problem

Cloud steer delivery needs negotiated capability and idempotent retry behavior.

Changes

Adds protocol negotiation, in-flight delivery deduplication, and safe normal-turn fallback.

[tatoalo]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat(cloud): expose steering controls for cloud sessions #3541
• fix(cloud): hydrate resumed transcripts atomically #3540
• fix(cloud): reconcile resumed events by leaf identity #3539
• feat(cloud): negotiate idempotent steer delivery #3538 👈 (View in Graphite)
• feat(cloud): add native steering to agent adapters #3537
• main

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit a04d2b1.}

[posthog]

👋 Visual changes detected for this PR.

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If these changes are unexpected, they may be caused by a flaky test or a broken snapshot on master. Don't approve — rerun the job or wait for a fix.

[stamphog]

This is a T1d-complex PR (741 lines, 7 files) with zero reviews, touching core agent protocol semantics: new concurrent delivery deduplication via inFlightMessageDeliveries, a counter-based owned-turn tracker for steer eligibility, and a behavioral policy change to sessionSupportsNativeSteer that enables cloud steering when explicitly negotiated. The code appears internally consistent and well-tested, but the concurrency state and protocol-level changes warrant at least one domain expert sign-off before auto-approval.