Executes declarative pipeline actions for :flow workflows.

For now, a flow pipeline is a list of steps under spec.pipeline (or spec.components), where each step can reference a capability handle:

pipeline: [
  %{type: :capability, handle: "my.capability"},
  ...
]

Each capability receives %{event, payload, current_state: nil, context} and may return:

• %{context: %{...}} to merge context updates
• %{result: any} to replace the carried payload/result