Build every judge where no pull request can reach, and never read mesh-lab at a release

Review of #329: a judge built with HOME in the workspace runs git under a
configuration a pull request's script can plant, so a hook could rewrite the
judge's sources before they compile and the result be kept under an honest key
(issue 482, issue 704). Every judge now builds with its own HOME, configuration
and Go cache outside the workspace, no git configuration and no VCS stamping.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmd/mesh-controller/release_branch_test.go

@@ -261,3 +261,13 @@ func TestATrunkIsTheBranchTheRepositoryFollows(t *testing.T) {
 		}
 	}
 }
+
+// trunkOf: the branch a source follows, and main when it records none or records a commit (issue 215's rule).
+func TestATrunkIsMainWhenTheSourceRecordsNone(t *testing.T) {
+	for ref, want := range map[string]string{"": "main", "main": "main", "master": "master", "nox-mesh": "nox-mesh",
+		"0123456789abcdef0123456789abcdef01234567": "main", "  ": "main"} {
+		if got := trunkOf(inventory.Source{Ref: ref}); got != want {
+			t.Errorf("a source recording %q follows %q, not %q", ref, got, want)
+		}
+	}
+}