Document prompt requirement inspection

docs/consumers/pkg-promptkit.md

@@ -154,6 +154,50 @@ rendered messages, and use prepared execution when that work must remain tied
 to later execution. The method's [GoDoc](../../engine.go) owns exact fields,
 hash, ownership, and error semantics.
 
+## Inspect Requirements Before Preparation
+
+Use [`Engine.InspectRequirements`](../../requirements.go) when the application
+needs the prompt's recognizable input and caller-variable dependencies for a
+particular context before it has input artifacts. Unlike `InspectPrompt`, it
+also walks captured template syntax and selects a profile identity; unlike
+`Prepare`, it does not load artifacts or render messages.
+
+```go
+requirements, err := engine.InspectRequirements(ctx, promptkit.RequirementInspectionRequest{
+	PromptID: "meeting.summary",
+	Vars:     map[string]string{"audience": "project leads"},
+})
+if err != nil {
+	return err
+}
+
+for _, input := range requirements.Inputs {
+	if input.Kind == promptkit.RequirementRequired {
+		// Plan to bind this input before preparation.
+	}
+	if input.Kind == promptkit.RequirementOptional && input.Explanation != "" {
+		// A recognizable reference may occur only on a known skipped path.
+	}
+}
+for _, variable := range requirements.Variables {
+	if variable.Kind == promptkit.RequirementRequired && !variable.Supplied {
+		// The key is required even though this inspection did not receive it.
+	}
+}
+if !requirements.Complete {
+	// Preserve the partial inventory and account for its discovery issues.
+}
+```
+
+Optional entries can include recognizable inputs and variables found only in
+inactive branches. A missing required variable is reported separately from
+discovery completeness: missing a value used only for direct text substitution
+does not by itself make the inventory incomplete. Conversely, an unresolved
+dependency-bearing branch can make discovery incomplete while retaining known
+conditional entries. Do not treat an empty partial inventory as proof that no
+input bindings are needed. See the method's [GoDoc](../../requirements.go) for
+the exact classifications, supported syntax, ownership, and error contract.
+
 ## Prepare Without Model Execution
 
 [`Engine.Prepare`](../../engine.go) resolves the selected prompt and profile,