Review in Linear: opt-in action like "Open on GitHub"

[Trosterud]

Problem

Linear now does great PR reviews. Diffs, inline comments, checks and approvals, everything, synced with GitHub directly (docs). Teams that review this way have no route from T3 Code into Linear's review. They open the PR on GitHub, then find it again in Linear.

Proposal

An off-by-default toggle, Settings → Source Control → GitHub → Review in Linear. When it's on, github.com pull requests offer Review in Linear next to Open on GitHub:

• in the PR detail ⋯ menu;
• in the right-click menu on a PR number, both in the list and on the detail;
• in mobile's PR menu and the list's long-press menu.

The link needs no Linear workspace, API token or new integration. Linear documents the swap github.com/owner/repo/pull/N → linear.review/owner/repo/pull/N, and its redirect takes the reader to the review in their own workspace. Opening the link is the whole feature.

Scope

• Only github.com PRs get the action, since Linear's redirect is documented for github.com URLs. Enterprise hosts and other hosts never show it.
• The setting is a field in GitHub's existing host settings (sourceControlHosts.github.reviewInLinear). That means no new top-level setting, and mobile reads it from the server it is already connected to.
• No server behavior changes, no new contracts, no new dependencies.
• It is not an "Open in" editor. That picker launches local apps on the thread's folder, while this opens a pull request.