Document prepared execution workflow

docs/consumers/pkg-promptkit.md

@@ -44,7 +44,9 @@ validation, and profile precedence are defined by the
 
 [`Engine.Prepare`](../../engine.go) resolves the selected prompt and profile,
 loads inputs and any structured-output schema, and renders messages without
-calling a model client:
+calling a model client. Choose it when the prepared value is the final
+inspection or persistence result and no later execution must be tied to that
+exact snapshot:
 
 ```go
 prepared, err := engine.Prepare(ctx, promptkit.RunRequest{
@@ -61,12 +63,48 @@ shows a complete runnable setup with a prompt file, in-memory profile, and
 inline input. Exact request requirements and prepared-result fields belong to
 the [`RunRequest` and `PreparedRun` GoDoc](../../types.go).
 
+## Prepare Now And Execute The Same Snapshot Later
+
+Use [`Engine.PrepareExecution`](../../engine.go) when an application must
+inspect or persist preflight details before deciding whether to start model
+work, while ensuring that later execution uses those exact rendered messages,
+inputs, target settings, and validation resources:
+
+```go
+preparedExecution, err := engine.PrepareExecution(ctx, promptkit.RunRequest{
+	PromptID: "meeting.summary",
+	Inputs: map[string]promptkit.ArtifactRef{
+		"note": promptkit.Inline("Synthetic meeting notes"),
+	},
+})
+if err != nil {
+	return err
+}
+defer preparedExecution.Discard()
+
+details := preparedExecution.Details()
+// Inspect or persist an application-selected safe subset of details.
+
+result, err := engine.RunPrepared(ctx, preparedExecution)
+```
+
+Preparation does not call the model or reserve backend capacity.
+`RunPrepared` executes from the retained snapshot rather than reloading
+consumer sources. The handle is opaque in-process state, while `Details`
+contains rendered content and remains subject to the application's data
+handling policy. The
+[`PreparedExecution` and method GoDoc](../../prepared_execution.go) and
+[engine operation GoDoc](../../engine.go) own exact lifecycle, engine-binding,
+credential, cancellation, timing, and error semantics.
+
 ## Execute And Validate
 
 [`Engine.Run`](../../engine.go) performs the same preparation, invokes the
 configured model client, classifies the generated artifact, and validates the
-content. A completed content check may return `ValidationFailed` in the result;
-an operational inability to validate returns an error.
+content in one call. Choose it when the application does not need a preflight
+boundary tied to the eventual execution. A completed content check may return
+`ValidationFailed` in the result; an operational inability to validate returns
+an error.
 
 The maintained
 [offline execution example](../../examples/go-library/run/main.go) injects a