feat(threads): dismiss async questions without replying

[t3dotgg]

Problem

An async question from the agent could only be closed by answering every part of it or by settling the thread. If you disagreed with the questions, or the agent had already finished the work, there was no way to release the composer without sending another message and spinning the agent back up. Reported in #9873.

Fix

Add a thread.user-input.dismiss command. The server records the question as resolved with a "User input dismissed" activity. No message is sent and no turn starts. Native callback questions cannot be dismissed this way because the provider is blocked waiting on a reply, so the server rejects those and the clients hide the control for them.

Web shows an X on the question card header. Mobile shows a "Dismiss without answering" action under Submit. Both only appear for async questions.

Surfaces covered: contracts, server decider and engine, client-runtime, web, mobile, Codex user doc. Tests cover the decider paths, the dismissible flag derivation, and the web control gating.

Closes #9873

Created by Claude Fable 5.1 in Claude Code.

[macroscopeapp]

Approvability

Verdict: Would Approve

Macroscope's review found this PR approvable — This adds a focused, user-invoked dismissal path for async questions while preserving existing answer behavior and rejecting dismissal for native callbacks. The additive command and UI plumbing are covered by targeted tests and do not alter product defaults or static-analysis settings.

Not approved because:

• Monthly spending limit reached (workspace setting). Approvability relies on correctness review in order to determine eligibility

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[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.6 KiB	13.6 KiB	−53 B (−0.4%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.0 KiB	7.0 KiB	+2 B (+0.0%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.6 KiB	6.5 KiB	−55 B (−0.8%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	57.1 KiB	57.0 KiB	−88 B (−0.2%)	66.4 KiB	✅
Codex	Live turn messages	10	8	−2 (−20.0%)	21	✅
Claude	Total thread wire	13.6 KiB	13.6 KiB	+49 B (+0.4%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.0 KiB	7.1 KiB	+4 B (+0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.5 KiB	6.6 KiB	+45 B (+0.7%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	57.8 KiB	57.9 KiB	+88 B (+0.1%)	66.4 KiB	✅
Claude	Live turn messages	8	10	+2 (+25.0%)	21	✅

Baseline: 252df77 · PR result: 576aac2 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 113.9 KiB
• Claude decoded thread snapshot: 114.6 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.