Hoist deterministic repeated agent work into workflow steps

Follow-up extracted from #1061. Design a current-architecture mechanism for moving deterministic, repeatedly observed agent work into generated `steps` or `post-steps`. Build on the typed IR and generated-shell binding model now on `main`; keep the initial scope independent from automatic source mutation.

Start by tracing the typed IR and generated-shell binding model on main. Determine where generated steps and post-steps are represented, then define a current-architecture mechanism for promoting deterministic repeated agent work without automatic source mutation. Done means the design and its implementation boundaries are documented clearly enough to review.

Written by the indexing model from the issue text.