Check a delivery group's member beside its siblings' merges, judged by the group's controller (issue 867, ADR 0382)

A mesh-host change and a mesh-controller change that need each other could not both pass their own checks: each read the other at its base. The check now asks mesh-delivery for the group's open siblings into a base of the same name, reads each at its head merged onto its base, builds the judge from a controller sibling's merge in the isolated judge environment keyed on its inputs, and names the siblings in the verdict. With no answer from mesh-delivery the verdict says it could not learn them; a pull request with no sibling is checked as before. TestReplay867 replays the failure.

Co-Authored-By: Claude <noreply@anthropic.com>

cmd/mesh-controller/checks.go

@@ -180,6 +180,8 @@ func (f following) PullUpdated(ctx context.Context, p link.PullUpdated) error {
 		return err
 	}
 	request.Check.Plan = &plan
+	// **The group's other open pull requests are read beside it** (novox/hq ADR 0382 §1).
+	withSiblings(ctx, &request, p, entries)
 	seat := buildSeatHeld(ctx)
 	ask, err := askOverOn(seat)
 	if err != nil {
@@ -600,6 +602,7 @@ func checkedOf(result link.BuildResult) link.Checked {
 		c.Built = result.Check.Built
 		c.Release, c.JudgedBy, c.BesideAt = result.Check.Release, result.Check.JudgedBy, result.Check.BesideAt
 		c.Validator = result.Check.Validator
+		c.Siblings, c.SiblingsUnknown, c.JudgeFrom = result.Check.Siblings, result.Check.SiblingsUnknown, result.Check.JudgeFrom
 		if c.Gate == nil {
 			// A build seat from before the layers: its verdict is the gate's.
 			c.Gate = &link.CheckLayer{Verdict: c.Verdict, Summary: c.Summary}