feat(executor): MCP-oriented planning preamble, applied by ContainerRunner (Phase 2)

Rewrites the planning preamble to point the agent at the four MCP tools
(ask_user/report_summary/spawn_subtask/record_progress) instead of the
CLAUDOMATOR_QUESTION_FILE / CLAUDOMATOR_SUMMARY_FILE conventions: ask_user ends
the turn, report_summary replaces the summary file, spawn_subtask replaces the
POST-to-/api/tasks planning step. Git discipline is retained.

ContainerRunner previously applied no preamble at all; it now prepends this
preamble (via buildAgentInstructions) when the MCP back-channel is active and
skip_planning is false, so the in-container agent is actually guided to use the
tools. Gemini agents (no MCP) are unaffected.

https://claude.ai/code/session_01SESwn7kQ7oP62trWw6pc39

1 files changed, 20 insertions, 10 deletions

diff --git a/internal/executor/preamble_test.go b/internal/executor/preamble_test.go
index 5c31b4f..77a8fca 100644
--- a/internal/executor/preamble_test.go
+++ b/internal/executor/preamble_test.go
@@ -5,27 +5,37 @@ import (
 	"testing"
 )
 
-func TestPlanningPreamble_ContainsFinalSummarySection(t *testing.T) {
-	if !strings.Contains(planningPreamble, "## Final Summary (mandatory)") {
-		t.Error("planningPreamble missing '## Final Summary (mandatory)' heading")
+func TestPlanningPreamble_ReferencesMCPTools(t *testing.T) {
+	for _, tool := range []string{"ask_user", "report_summary", "spawn_subtask", "record_progress"} {
+		if !strings.Contains(planningPreamble, tool) {
+			t.Errorf("planningPreamble should mention the %q tool", tool)
+		}
 	}
 }
 
-func TestPlanningPreamble_SummaryUsesFileEnvVar(t *testing.T) {
-	if !strings.Contains(planningPreamble, "CLAUDOMATOR_SUMMARY_FILE") {
-		t.Error("planningPreamble should instruct agent to write summary to $CLAUDOMATOR_SUMMARY_FILE")
+func TestPlanningPreamble_NoFileEscapeHatches(t *testing.T) {
+	for _, marker := range []string{"CLAUDOMATOR_SUMMARY_FILE", "CLAUDOMATOR_QUESTION_FILE"} {
+		if strings.Contains(planningPreamble, marker) {
+			t.Errorf("planningPreamble should no longer reference %s", marker)
+		}
 	}
 }
 
-func TestPlanningPreamble_SummaryInstructsEchoToFile(t *testing.T) {
-	if !strings.Contains(planningPreamble, `"$CLAUDOMATOR_SUMMARY_FILE"`) {
-		t.Error("planningPreamble should show example of writing to $CLAUDOMATOR_SUMMARY_FILE via echo")
+func TestPlanningPreamble_AskUserEndsTurn(t *testing.T) {
+	if !strings.Contains(planningPreamble, "end your turn") {
+		t.Error("planningPreamble should instruct the agent to end its turn after ask_user")
 	}
 }
 
 func TestPlanningPreamble_GitDiscipline_ForbidsAbsolutePaths(t *testing.T) {
-	// Agents must not bypass the sandbox by using absolute project paths in git commands.
 	if !strings.Contains(planningPreamble, "do NOT use absolute paths") {
 		t.Error("planningPreamble should warn agents not to use absolute paths in git commands")
 	}
 }
+
+func TestWithPlanningPreamble_PrependsToInstructions(t *testing.T) {
+	got := withPlanningPreamble("DO THE THING")
+	if !strings.HasPrefix(got, planningPreamble) || !strings.HasSuffix(got, "DO THE THING") {
+		t.Error("withPlanningPreamble should prepend the preamble to the instructions")
+	}
+}