Implement support for OpenRouter sticky routing via a session_id variable

internal/usecase/runner.go

@@ -90,7 +90,7 @@ func (r *Runner) Run(ctx context.Context, req domain.RunRequest) (*domain.RunRes
 	}
 
 	genResp, err := r.llm.Generate(ctx, domain.GenerateRequest{
-		Prompt:           domain.RenderedPrompt{Messages: prepared.Messages},
+		Prompt:           domain.RenderedPrompt{SessionID: prepared.SessionID, Messages: prepared.Messages},
 		Target:           prepared.EffectiveModelParams,
 		StructuredOutput: prepared.StructuredOutput,
 	})
@@ -233,6 +233,7 @@ func (r *Runner) Prepare(ctx context.Context, req domain.RunRequest) (*domain.Pr
 		OutputContract:       effectiveContract,
 		StructuredOutput:     structuredOutput,
 		InputHashes:          inputHashes,
+		SessionID:            renderedPrompt.SessionID,
 		RenderedPromptHash:   hashRenderedPrompt(*renderedPrompt),
 		Messages:             renderedPrompt.Messages,
 		StartTime:            start,
@@ -424,6 +425,11 @@ func resolveOutputContract(def *domain.PromptDefinition, override *domain.Output
 
 func hashRenderedPrompt(p domain.RenderedPrompt) string {
 	var b strings.Builder
+	if p.SessionID != "" {
+		b.WriteString("session_id=")
+		b.WriteString(p.SessionID)
+		b.WriteString("\n---\n")
+	}
 	for _, msg := range p.Messages {
 		b.WriteString(msg.Role)
 		b.WriteByte('\n')