Issue #3606902: Interaction chains, render-only messages, progress

Implements the interaction-chains model. See issue #3606902 and docs/interaction-chains.md.

This MR is the first increment: the renderOnly flag on the #[Interaction] attribute, render-only gating of the terminal end-node screen, and the design doc. Subsequent increments add the token chain_id / has_interaction fields, chain-scoped dispatch with message aggregation, the sticky flag, the per-workflow old-link setting, and the progress bar.