fix(pair): pair_await lists the latest turn's files only

[rynfar]

What

pair_await now lists the files changed by the executor's latest turn, not by every turn it has ever run.

Why

A pair keeps one executor for the life of the lead thread, so its checkpoints accumulate. filesChanged aggregated all of them, which meant that by the fifth brief the lead was handed the files of the first four again. That is noise in the one result the lead is supposed to review carefully, and tokens it pays for on every await.

How

• latestTurnCheckpoints(checkpoints, latestTurnId) in pair/logic.ts keeps the checkpoints whose turnId is the executor's latest turn; a turn that changed nothing has no checkpoint and shows no files.
• pair_await aggregates over those. turnCount still counts every completed turn.
• The fan-out delegated_thread_result is unchanged: a fan-out child normally runs one task.

Verification

• vp test run on the pair and delegation toolkits and apps/server/src/orchestration, on this branch rebased over feat(pair): pair without the Pylon delegation setting #648: 926 passed. New: the pure rule, and a handler test with two turns where only the second turn's file is listed and turnCount is 2. Both were written first and seen failing.
• Typecheck t3: 0 errors. Format clean.
• Not run live.

Part of #622.

Model: Claude Fable 5.1, in Pylon via the Claude Code harness.

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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	14.0 KiB	13.9 KiB	−31 B (−0.2%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.2 KiB	7.2 KiB	+5 B (+0.1%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.8 KiB	6.7 KiB	−36 B (−0.5%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	58.0 KiB	58.0 KiB	−44 B (−0.1%)	66.4 KiB	✅
Codex	Live turn messages	10	9	−1 (−10.0%)	21	✅
Claude	Total thread wire	14.0 KiB	14.0 KiB	−2 B (−0.0%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.2 KiB	7.2 KiB	+1 B (+0.0%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.8 KiB	6.8 KiB	−3 B (−0.0%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	58.9 KiB	58.9 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	10	10	0 (0.0%)	21	✅

Baseline: 933d6f7 · PR result: e2c521c · 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: 115.6 KiB
• Claude decoded thread snapshot: 116.3 KiB

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