fix: rewrite OpenCode identity sentence to Claude Code

backend/internal/service/gateway_sanitize_test.go

@@ -0,0 +1,20 @@
+package service
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestSanitizeOpenCodeText_RewritesCanonicalSentence(t *testing.T) {
+	in := "You are OpenCode, the best coding agent on the planet."
+	got := sanitizeOpenCodeText(in)
+	require.Equal(t, strings.TrimSpace(claudeCodeSystemPrompt), got)
+}
+
+func TestSanitizeOpenCodeText_RewritesOpenCodeKeywords(t *testing.T) {
+	in := "OpenCode and opencode are mentioned."
+	got := sanitizeOpenCodeText(in)
+	require.Equal(t, "Claude Code and Claude are mentioned.", got)
+}