fix: preserve spawn issue context field

backend/internal/ports/session.go

@@ -18,6 +18,9 @@ type SpawnConfig struct {
 	Harness   domain.AgentHarness
 	Branch    string
 	Prompt    string
+	// IssueContext is optional pre-fetched tracker context for the task prompt.
+	// Standing rules stay in SystemPrompt; issue facts belong to the user task.
+	IssueContext string
 	// DisplayName is the user-facing sidebar label. Empty falls back to the
 	// session id in the read model (e.g. orchestrator sessions).
 	DisplayName string