feat(web): cite a stretch of the transcript, tool calls included - #476

Citations could only quote text inside one assistant response, so there was no way to point an agent at a run of the conversation: what it said, the commands it ran, and what it concluded.

This adds transcript citations:

• Drag across messages. A selection that runs from one message to another cites everything between: user messages, assistant responses, and the tool calls the drag passes over. Only the selected part of the first and last response is kept. Tool rows can't hold a text selection, so the ones included get a tint.
• Shift-click tool calls. Shift-clicking a tool call picks it instead of expanding it. On its own it cites that call; with text selected it extends the quote to it. While picking, shift-clicking more tool calls or messages moves the end. A Cite button sits above the composer, and the pick survives scrolling. Escape, ✕ or a plain click clears it.
• Chip and agent context. The chip reads what the excerpt spans ("1 message, 4 tool calls"), and its preview shows the excerpt as a short transcript. Agents receive the same rendering: each tool call's name, label, status and trimmed detail, plus ordered segments naming each item's id. Items hidden in collapsed tool groups or folded turns are included; reasoning is left out.
• Go to source. Scrolls to the first cited item and briefly tints the rest.

How

AssistantCitation gains an optional segments array (message or tool-call ids in timeline order); text/markdown hold the rendered excerpt. Extending the existing type rather than adding a new one means every consumer (composer chips, queue and stash previews, titles, find, mobile, provider expansion, sending to another thread) handles excerpts without changes. Old citation links are byte-identical. A client without this change won't recognise a link carrying segments and will show it as a plain link.

Segments are built from the timeline entries, not the DOM, because LegendList recycles rows out of order and unmounts off-screen ones (transcriptCitation.logic.ts). Only the two ends of a drag are read from the DOM.

Screenshots

Dragging from one response to the next, across "Ran 4 commands":

Before	After

Before, a cross-row selection offers no Cite button. After, the commands in the range are tinted (reasoning rows are not) and Cite appears.

Shift-clicking a command and then the response below it picks whole items, with the action above the composer:

The chip and its comment panel, then the chip's hover preview:

Comment panel	Preview

Verification

• New unit tests for the excerpt builder and the segments link format, and a browser test for drag, shift-click and Escape. Temporarily removing the shift-click interception makes the browser test fail.
• Exercised on a seeded dev server with Playwright (the screenshots above). This turned up the Cite button rendering under the composer overlay, fixed in the second commit.
• check:changed is clean for this change. Its remaining failures exist without it: knip's unused exports, a server typecheck error in ProviderInstanceRegistry.test.ts, and two "opens a nested diff" tests in MessagesTimeline.browser.test.tsx.
• Docs: docs/user/composer.md and a new section in docs/fork-differences.md.

Not covered: mobile only renders excerpts as text (it doesn't create citations, as before), and I didn't test the desktop app separately from web.

Claude Opus 5.5 in T3 Code (Claude Code harness).

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