harness: composer ↑/↓ arrow history does not work while agent is busy

[btipling]

Summary

PR #686 shipped composer arrow-key history, but pressing ↑ or ↓ while the agent is busy (thinking/inference running) does nothing — no navigation to prior user messages.

Expected

Pressing ↑ when the composer is empty and the agent is busy should load the newest user message, same as when idle.

Actual

↑/↓ are ignored while busy is true. The guard in ui.zig reads:

if (!busy and state.queue_editing_index == null) {
    // ... history intercept ...
}

This deliberately blocks history during inference. The original intent (per the plan #667 and adversarial-review feedback) was that Busy should NOT block history — the !busy guard was a reviewer recommendation that the implementer accepted, but it produces the wrong operator experience.

Repro

1. Start a long-running agent turn (send a message)
2. While the agent is busy (spinner visible), press ↑
3. Nothing happens — composer stays empty

Notes

• queue_editing_index guard is correct and should stay
• Only the !busy guard needs removal
• No other behavior changes needed — the fingerprint/draft/hydrate logic is unchanged

Related

• PR feat(harness): composer ↑/↓ arrow-key history — navigate prior user messages #686 — feat(harness): composer ↑/↓ arrow-key history
• Merge SHA ebe9354
• Current harness at 4bc831b

[btipling]

Completed by #707 (plan #704).

Composer ↑/↓ history no longer gated on !busy. Guard is only queue_editing_index == null (ui.zig ~648). Docs: docs/harness-limits.md — “Works while Busy.” Merged to main 2026-08-19.