Diagnose persistent mergeable=UNKNOWN on TextureHQ/mono PRs

mergeable=UNKNOWN + mergeStateStatus=UNKNOWN has appeared on PRs #14712 and #15083 despite all checks green and review approved. Compare fleet token vs personal token gh pr view output on #15083, query GitHub REST API pulls/15083 mergeability, and document whether this is token-scope limitation, repo size/computation deferral, or branch-protection behavior.