feat(FN-742): add describeModel helper and log model in agent creation sites

- Add describeModel() helper in pi.ts to format provider/model info for logging
- Log resolved model details in executor, reviewer, and triage agent creation
- Update executor and reviewer to call describeModel before session start
- Add unit tests for describeModel covering all input combinations
- Fix test mocks to account for new describeModel dependency

packages/engine/src/pi.ts

@@ -22,6 +22,17 @@ export interface AgentResult {
   session: AgentSession;
 }
 
+/**
+ * Extract a human-readable model description from an AgentSession.
+ * Returns `"<provider>/<modelId>"` (e.g. `"anthropic/claude-sonnet-4-5"`)
+ * or `"unknown model"` when the session has no model set.
+ */
+export function describeModel(session: AgentSession): string {
+  const model = session.model;
+  if (!model) return "unknown model";
+  return `${model.provider}/${model.id}`;
+}
+
 export interface AgentOptions {
   cwd: string;
   systemPrompt: string;