Expose backend identity and capacity errors

internal/format/prepared_run.go

@@ -93,6 +93,9 @@ func (textPreparedRunFormatter) Format(prepared *promptkit.PreparedRun) ([]byte,
 	fmt.Fprintf(&b, "prompt: %s\n", prepared.PromptID)
 	fmt.Fprintf(&b, "prompt_version: %s\n", prepared.PromptVersion)
 	fmt.Fprintf(&b, "selected_profile_id: %s\n", prepared.SelectedProfileID)
+	if prepared.SelectedBackendID != "" {
+		fmt.Fprintf(&b, "selected_backend_id: %s\n", prepared.SelectedBackendID)
+	}
 	if prepared.PromptHash != "" {
 		fmt.Fprintf(&b, "prompt_hash: %s\n", prepared.PromptHash)
 	}

internal/format/prepared_run_test.go

@@ -22,6 +22,7 @@ func TestTextFormatterIncludesPreparedRunDetails(t *testing.T) {
 		"prompt: prompt.id",
 		"prompt_version: v1",
 		"selected_profile_id: local-fast",
+		"selected_backend_id: local",
 		"endpoint: http://llm/v1",
 		"model: gpt-test",
 		"temperature: 0.4",
@@ -346,7 +347,9 @@ func samplePreparedRun() *promptkit.PreparedRun {
 		PromptVersion:     "v1",
 		PromptHash:        "prompt-hash",
 		SelectedProfileID: "local-fast",
+		SelectedBackendID: "local",
 		EffectiveModelParams: promptkit.ExecutionTarget{
+			BackendID:       "local",
 			Endpoint:        "http://llm/v1",
 			Model:           "gpt-test",
 			Temperature:     0.4,