feat(evals): evaluate subagent orchestration (parent → child workflow) #8570

Problem

The eval suite covers the tool loop, the executor's single-block paths, context
assembly, and model comparison. It does not cover orchestration: a parent
workflow invoking a child, parallel fan-out and merge, or a loop subagent. Those
paths are where result propagation, partial failure, and depth limits go wrong,
and nothing evaluates them end to end.

Proposal

Add an agent-orchestration suite that runs parent → child workflows through the
real DAGExecutor.

• Parent: Start → Workflow block that invokes a named child.
• Child: readWorkflowDefinitionAsExecutor is mocked to return a child
  definition (e.g. Start → Agent, provider mocked); the child executor runs for
  real, so the parent→child boundary is exercised, not simulated.
• Reuse the central @sim/testing mocks already used by
  executor/handlers/workflow/workflow-handler.test.ts (logging session,
  billing attribution, permissions, auth-internal, snapshot, custom-block ops).

First scenarios

1. Child output reaches the parent — the child returns a value; the parent's
   Workflow block output maps it; the run succeeds.
2. Child failure fails the parent — the child errors; the parent surfaces the
   mapped error and does not report success.
3. Call-chain depth limit — a recursive child chain stops at the limit
   instead of looping forever.

Scope / difficulty

The Workflow block handler mixes persistence, billing, logging sessions,
delegation tokens, and custom-block policy; its unit test mocks ~15
collaborators. This eval should cover only the DAG-level integration the unit
test cannot, and reuse the central mocks rather than re-deriving them.

Acceptance criteria

• A parent invokes a child and the child's output reaches the parent result
• A failed child fails the parent with the mapped error
• Runs through the real DAGExecutor, not the handler in isolation
• Results appear in a report and the README documents the suite
• A negative check proves the assertions fail when child output is dropped