State the seal rules a JSON shape cannot show

Run 9's research seal was refused for writing "F1" as a finding id. The rule
is real and the message was precise, but no brief had ever stated it: the
shape block shows keys and types, and a format constraint is neither.

The agent recovered in fifteen seconds, so this cost one boundary rather than
a run. It is still a refusal nobody had to earn, and the same shape hid F38 a
few runs ago.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CVbaKucEYBjMqVeUgJUsc1

internal/agentctx/agentctx_test.go

@@ -696,3 +696,14 @@ func TestAllPhasesStaleIsNotReportedAsFinished(t *testing.T) {
 		t.Errorf("a plan verified at HEAD was not reported as finished:\n%s", current)
 	}
 }
+
+// The shape cannot show a format rule, and run 9 was refused for writing "F1"
+// as a finding id against a constraint no brief had ever stated.
+func TestResearchBriefStatesTheFindingIDRule(t *testing.T) {
+	brief := phaseRequestBrief(domain.WorkPhaseResearch)
+	for _, want := range []string{"lowercase letters", "at most 64 characters", "unique"} {
+		if !strings.Contains(brief, want) {
+			t.Errorf("research brief never states %q", want)
+		}
+	}
+}