feat: require agents to write a final summary before exiting

Add a mandatory '## Final Summary' section to planningPreamble
instructing agents to output a 2-5 sentence summary paragraph
(headed by '## Summary') as their last output before exiting.
Adds three tests to verify the section and its required content
are present in the preamble.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

2 files changed, 37 insertions, 0 deletions

diff --git a/internal/executor/preamble.go b/internal/executor/preamble.go
index e50c16f..1993361 100644
--- a/internal/executor/preamble.go
+++ b/internal/executor/preamble.go
@@ -46,6 +46,17 @@ The sandbox is rejected if there are any uncommitted modifications.
 
 ---
 
+## Final Summary (mandatory)
+
+Before exiting, write a final summary paragraph (2-5 sentences) as your last output. Start it with "## Summary" on its own line. Describe:
+- What was accomplished
+- Key decisions made
+- Any issues or follow-ups needed
+
+This summary will be extracted and displayed in the task UI.
+
+---
+
 `
 
 func withPlanningPreamble(instructions string) string {
diff --git a/internal/executor/preamble_test.go b/internal/executor/preamble_test.go
new file mode 100644
index 0000000..448ad3a
--- /dev/null
+++ b/internal/executor/preamble_test.go
@@ -0,0 +1,26 @@
+package executor
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestPlanningPreamble_ContainsFinalSummarySection(t *testing.T) {
+	if !strings.Contains(planningPreamble, "## Final Summary (mandatory)") {
+		t.Error("planningPreamble missing '## Final Summary (mandatory)' heading")
+	}
+}
+
+func TestPlanningPreamble_SummaryRequiresMarkdownHeader(t *testing.T) {
+	if !strings.Contains(planningPreamble, `Start it with "## Summary"`) {
+		t.Error("planningPreamble does not instruct agent to start summary with '## Summary'")
+	}
+}
+
+func TestPlanningPreamble_SummaryDescribesRequiredContent(t *testing.T) {
+	for _, phrase := range []string{"What was accomplished", "Key decisions made", "Any issues or follow-ups"} {
+		if !strings.Contains(planningPreamble, phrase) {
+			t.Errorf("planningPreamble missing required summary content description: %q", phrase)
+		}
+	}
+}