agent: Per-step model hints in workflow YAML

[krokoko]

Component

Agent (Python runtime)

Describe the feature

Allow workflow YAML steps to declare an optional model override so different phases of a task can use different Bedrock models without defining separate workflows.

Example (single run_agent step permitted today):

steps:
  - { kind: clone_repo, name: setup }
  - { kind: run_agent, name: implement, model: { id: anthropic.claude-sonnet-4-6 } }
  - { kind: verify_build, name: build, gate: regression_only }

Resolution follows WORKFLOWS.md model selection: Blueprint allow-list bounds all choices; per-task API override remains highest unless allow_task_override: false.

Use case

Model fit is task-phase-specific. Today agent_config.model applies workflow-wide. Per-step model on the lone run_agent step documents intent and prepares the schema for future multi-step agent graphs.

Distinct from #439 (runtime per-turn adaptive router) — this is declarative workflow data, not a heuristic.

Proposed solution

1. Add optional model: { id, allow_task_override? } to step objects in workflow JSON Schema (run_agent only initially).
2. Validator: step model.id must be on platform/Blueprint allow-list.
3. Runner: merge step-level model over workflow.agent_config.model over Blueprint default in _handle_run_agent.
4. Telemetry: record resolved model per step on TaskEvents / OTEL spans.
5. Tests and WORKFLOWS.md update.

Other information

• Related: feat(agent): adaptive / complexity-aware model router #439 (per-turn router — complementary), ADR-014 / feat(tasks): capability-driven tasks — declarative steps, non-coding workflows, migrate existing task types #248 workflows.

Acknowledgements

• I may be able to implement this feature
• This might be a breaking change (schema addition only — backward compatible if optional)