feat(FN-5205): add workflow-step test mode dispatch and routing for mock pr

Implements workflow step test mode (FN-5205) by wiring mock dispatch, context forwarding, and routing through executor, merger, and mock provider, plus adding corresponding tests and docs. Also adds broad-scope triage heuristics to improve task-scope detection, touching triage.ts, triage-broad-scope

Fusion-Task-Id: FN-5205

Co-authored-by: Fusion (runfusion.ai) <noreply@runfusion.ai>
Fusion-Task-Id: FN-5205

packages/engine/src/agent-session-helpers.ts

@@ -280,6 +280,8 @@ export async function createResolvedAgentSession(
       runtimeContext: {
         ...runtimeOptions.runtimeContext,
         sessionPurpose,
+        workflowStepId: runtimeOptions.runtimeContext?.workflowStepId,
+        workflowStepTemplateId: runtimeOptions.runtimeContext?.workflowStepTemplateId,
       },
     }
     : runtimeOptions;