[LHagfoss]

What

A delegate started on a paired machine from another computer was nested under its parent here, and the machine's Devices view skips nested threads, so that machine listed none of its own delegates (#3441). It now keeps a row in the machine it runs on, with a from <thread> on <machine> hint that opens the parent; it is still nested under the parent. An empty machine says "No threads on this machine yet." with a New thread button that starts one there. The rule is crossMachineDelegates in src/state/sidebarGroups.ts.

For #3440 this PR changes how a delegate's answer reads, not why it was marked Failed:

• the row's preview is plain text (first prose line; a table-only answer reads as its cells, no pipes) instead of the raw first line;
• a failed row previews the turn's error, stored apart from the reply (Link.error), instead of the reply drawn in the error colour.

Not fixed: why the remote Claude Code delegate's link settled as failed after a full answer. A failed link comes only from the prompt request rejecting (runPrompt in src/state/store.ts treats any rejection, including a dropped machine connection, as a failed turn). I could not reproduce or confirm the cause from the code alone, so #3440 stays open.

Changelog

A delegate running on another machine now appears in that machine's thread list, and delegate rows show a plain-text preview of the answer.

Testing

• bun run check in apps/desktop: typecheck, lint, 2031 tests, build.
• New unit tests: crossMachineDelegates with a remote delegate whose parent is local (sidebarGroups.test.ts), and answerPreview (answerPreview.test.ts).

Closes #3441
Refs #3440