feat(web): list linked PRs in workspace card

[Bil0000]

A thread can link several pull requests, but the Workspace card showed only one.

Keep the current PR row as it was. Add Show N more to reveal the other linked PRs in stack order. Each revealed PR uses the existing row, so its checks and available action come from that PR. Collapsing the list removes the extra rows and their host reads.

Verified with 8 focused tests, web typecheck, targeted lint, and the running web app with four linked PRs. The app check covered collapsed and expanded views, passing and failing checks, Fix, Resolve, and a merged PR.

Expanding the list makes one detail request per revealed PR; the collapsed card does not.

Before:

After:

Built with GPT-5 in Codex.

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This PR adds an expandable linked-pull-request capability to workspace cards and causes each revealed row to use existing detail, check-refresh, and PR-action behavior. The change is localized and tested, but its new user-facing workflow and additional host reads warrant human review.

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[Bil0000]

CI needs a maintainer rerun: Test Server 2 failed in apps/server/src/vcs/GitVcsDriverCore.test.ts (keeps complete stats for files beyond the combined patch limit). This PR changes only three web files. I ran that exact test locally on the PR head and it passed. GitHub refused both failed-job rerun routes with 403 Must have admin rights to Repository. Could a maintainer rerun the failed job?