Publish only a commit on its module's trunk; post a pull request's change plan (hq ADR 0238)

One commit, one plan: a commit off the trunk — a pull request's head, a branch built by
hand, a rebuild or replay of one — is for checking. The build seat reads from its clone
which branches hold the commit, and the controller records and never registers a build
whose commit is not on the branch the module follows (the repository's default for a
new one), so nothing off the trunk can be sent.

A pull request's check now carries its change plan, computed by the planner: what a
merge would build in which order, what each machine would receive, and what is not an
ordinary send — the bus step, a module waiting for a person, a provider's consumers.

cmd/mesh-builder/main.go

@@ -293,6 +293,7 @@ func answer(ctx context.Context, publisher builder.Publisher, on, workspace stri
 			}
 			result.Against = built.Against
 			result.SourceFingerprint = built.Source
+			result.Trunk, result.OnTrunk, result.Branches = built.Trunk, built.OnTrunk, built.Branches
 			for _, r := range built.Read {
 				result.Read = append(result.Read, link.ReadRepository{Repository: r.Repository, Ref: r.Ref})
 			}