feat(engine): sequential workflow executor core (closes #22)

[leo-aa88]

Summary

Implements the sequential workflow execution engine from issue #22 and design doc sections 12.2 E, 13.3, 13.4, and 14.2: run workflows step-by-step with interpolation, policy, tools, agents, schema validation, and persistence.

New files (as scoped)

File	Purpose
execution.go	Executor, RunInput, Run orchestration
workflow.go	Workflow lookup, optional workflow input JSON Schema, workflow output interpolation
steps.go	Tool and agent step execution, agent output schema validation
timeout.go	Per-agent-step context.WithTimeout from Agent.constraints.timeoutSeconds
retries.go	Single retry when model errors wrap ErrTransientGeneration

Behavior

• Steps: exactly one of uses (tool) or agent per step; native/MCP/HTTP/mock tools go through tools.ToolExecutor; agents use models.ModelClient (registry or ModelResolve for tests).
• Interpolation: existing InterpolateWalk on each steps with` map.
• Policy: policy.Engine.Evaluator for the workflows spec.policy` name (budgets, structured output, tools, approvals).
• Agent prompt (MVP): system message = agent instructions; user message = JSON-marshaled with map.
• Agent output: response body must be a JSON object; if agent.spec.output.schema is set, schema.Validate runs (hard fail on invalid instance).
• Persistence: expects an existing runs row; updates run_steps, FinishRun, optional trace.Recorder (step.*, tool.*, model.*).
• Tool retries: unchanged — still driven by tool spec.retry inside MCP/HTTP runtimes.

Tests

• Integration: multi-step workflow — native echo tool then mock model agent; asserts final run output_json and trace events.
• Schema: agent returns JSON missing required summary; run ends failed.
• Retry helper: withAgentRetry invokes fn twice on transient error.

Verification

• make fmt
• make vet
• make test (-race)

Follow-ups (out of scope for #22)

• Wire agentctl run CLI, parallelism, conditionals, environment policy merge.
• Richer transient-error detection for real providers.

Closes #22

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