New POST /api/github/pulls/detail endpoint + GitHubClient.getPullRequestDetail (gh pr view --json comments,statusCheckRollup, REST fallback). On PR select, the preview fetches detail (cached per PR, body never blocked) and renders below the body: a Checks section (each check name + status pill success/failure/pending/neutral, linked to detailsUrl) and a Comments section (chronological, markdown via MailboxMessageContent). Issues tab unchanged.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>