Add PreparedRun model and render formatters (text/json) with tests

internal/domain/domain.go

@@ -75,6 +75,22 @@ type RunResult struct {
 	Error                error
 }
 
+// PreparedRun contains pre-LLM execution state from the prepare/render phase.
+// It must never include resolved API key values, model output, or validation data.
+type PreparedRun struct {
+	PromptID             string            `json:"prompt_id"`
+	PromptVersion        string            `json:"prompt_version,omitempty"`
+	PromptHash           string            `json:"prompt_hash,omitempty"`
+	SelectedProfileID    string            `json:"selected_profile_id"`
+	EffectiveModelParams ExecutionTarget   `json:"effective_model_params"`
+	InputHashes          map[string]string `json:"input_hashes,omitempty"`
+	RenderedPromptHash   string            `json:"rendered_prompt_hash"`
+	Messages             []RenderedMessage `json:"messages"`
+	StartTime            time.Time         `json:"start_time,omitempty"`
+	EndTime              time.Time         `json:"end_time,omitempty"`
+	DurationMS           int64             `json:"duration_ms,omitempty"`
+}
+
 // ArtifactRef represents a reference to an input artifact.
 type ArtifactRef struct {
 	Type ArtifactRefType
@@ -156,13 +172,13 @@ type OutputContract struct {
 
 // RenderedPrompt represents the prompt after template application.
 type RenderedPrompt struct {
-	Messages []RenderedMessage
+	Messages []RenderedMessage `json:"messages"`
 }
 
 // RenderedMessage is a single message in a rendered prompt.
 type RenderedMessage struct {
-	Role    string
-	Content string
+	Role    string `json:"role"`
+	Content string `json:"content"`
 }
 
 // GenerateRequest is the internal request passed to the LLM client.