Characterise today's STEP_ENTERED divergence between run and collect

[tobyhede]

Parent

#799. Design record: .scratch/render-execution-unit/spec.md.

What to build

Tests that pin what STEP_ENTERED carries today on each path, so the behaviour change that follows reads as a one-line diff. Green against unmodified code.

Three divergences to capture, all caused by collection-service building a StepEntryMetadata the CLI's builder fills differently:

• A substep with a description emits that description when entered via rundown run, and emits none when entered via rundown collect.
• prompted is composed from the persisted flag and the step's prompted-FOR kind on the loop path, but from the persisted flag alone on the collect path.
• substepId is populated from the raw cursor while isSubstep is derived from the resolved execution unit, so a cursor naming no live substep yields a populated substepId with isSubstep: false.

Follows the #790 pattern: characterise first, so the fix is visible as an assertion flipping.

Acceptance criteria

• Each of the three divergences is asserted against current behaviour and green.
• The prompted and substepId/isSubstep assertions state which value is the correct one in a comment, so the later flip is unambiguous.
• Existing pinned payloads in the collection-service suite are left as-is; this ticket adds, it does not edit.

Blocked by

None — can start immediately.