test: improve e2e test debugging by capturing stderr

Added stderr buffer to TestHarness to capture mothership output
when health check fails. This helps diagnose issues like port
conflicts during e2e test runs.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

mothership/tests/e2e/e2e_test.go

@@ -3,6 +3,7 @@
 package e2e
 
 import (
+	"bytes"
 	"context"
 	"encoding/json"
 	"errors"
@@ -42,6 +43,7 @@ type TestHarness struct {
 	MothershipURL string
 	APIURL        string
 	t             *testing.T
+	stderrBuf     *bytes.Buffer // Captured stderr for debugging
 }
 
 // NewTestHarness creates a new test harness
@@ -82,7 +84,8 @@ func (h *TestHarness) Start(ctx context.Context) error {
 		"TZ=UTC",
 	)
 	h.MothershipCmd.Stdout = io.Discard
-	h.MothershipCmd.Stderr = io.Discard
+	h.stderrBuf = &bytes.Buffer{}
+	h.MothershipCmd.Stderr = h.stderrBuf
 
 	if err := h.MothershipCmd.Start(); err != nil {
 		return fmt.Errorf("failed to start mothership: %w", err)
@@ -92,6 +95,11 @@ func (h *TestHarness) Start(ctx context.Context) error {
 
 	// Wait for health check
 	if err := h.WaitForHealth(ctx); err != nil {
+		// Log captured stderr for debugging
+		stderrStr := h.stderrBuf.String()
+		if stderrStr != "" {
+			h.t.Logf("Mothership stderr:\n%s", stderrStr)
+		}
 		h.Stop()
 		return fmt.Errorf("health check failed: %w", err)
 	}