todo: turn timer in chat, and a sub-agent spawn endpoint bounded by usage limits

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

todo/chat/_index.md

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+---
+theme: chat
+heading: "Chat interface"
+kind: theme
+---
+
+The chat surface itself — what a turn shows about itself while and after it runs.

todo/chat/show-how-long-a-turn-took-beside-its-cost.md

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+---
+id: chat/show-how-long-a-turn-took-beside-its-cost
+title: "Show how long a turn took, beside its cost and tokens"
+theme: chat
+status: open
+created: 2026-08-21
+date_source: stated
+order: 1
+---
+
+**Show how long a turn took, beside its cost and tokens.** Wanted 2026-08-21. A timer running from the
+start of each turn, and the elapsed time kept on the settled row — in the same place that already
+renders cost and token count, not somewhere new.
+
+That row is `MessageBubble.tsx:264`, and today it reads
+`Done · ${message.cost.totalUSD.toFixed(3)} · {inputTokens + outputTokens} tokens`. Duration is the
+third number that belongs there and is the only one missing.
+
+Two things already exist and should be reused rather than rebuilt. `formatDuration` is in the same file
+(~line 395), and the compaction row beside it already does exactly this job — its header comment says
+the elapsed time is "honest for the window that watched it happen, and the settled row carries the
+duration", which is the behaviour wanted here. What is missing is the field: `MessageCost` in
+`Chat/types.ts:12` carries `totalUSD` and the token counters but no duration, while
+`compact:done` carries `durationMs`. So the harness result has to bring it along, or the client has to
+time the turn itself and keep it on the settled row.