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, 22 insertions, 0 deletions

diff --git a/internal/executor/container_test.go b/internal/executor/container_test.go
index 86e95e2..3d0887c 100644
--- a/internal/executor/container_test.go
+++ b/internal/executor/container_test.go
@@ -54,6 +54,28 @@ func TestContainerRunner_BuildDockerArgs(t *testing.T) {
 	}
 }
 
+func TestBuildAgentInstructions(t *testing.T) {
+	t.Run("mcp enabled prepends preamble", func(t *testing.T) {
+		tk := &task.Task{Agent: task.AgentConfig{Instructions: "do the thing"}}
+		got := buildAgentInstructions(tk, true)
+		if !strings.HasPrefix(got, planningPreamble) || !strings.HasSuffix(got, "do the thing") {
+			t.Errorf("expected preamble + instructions, got %q", got)
+		}
+	})
+	t.Run("mcp disabled keeps raw instructions", func(t *testing.T) {
+		tk := &task.Task{Agent: task.AgentConfig{Instructions: "do the thing"}}
+		if got := buildAgentInstructions(tk, false); got != "do the thing" {
+			t.Errorf("expected raw instructions, got %q", got)
+		}
+	})
+	t.Run("skip planning keeps raw instructions even with mcp", func(t *testing.T) {
+		tk := &task.Task{Agent: task.AgentConfig{Instructions: "do the thing", SkipPlanning: true}}
+		if got := buildAgentInstructions(tk, true); got != "do the thing" {
+			t.Errorf("expected raw instructions when skip_planning, got %q", got)
+		}
+	})
+}
+
 func TestWriteMCPConfig(t *testing.T) {
 	dir := t.TempDir()
 	if err := writeMCPConfig(dir, "http://host.docker.internal:8484/mcp", "tok-abc"); err != nil {