Add structured Changes blocks to Crane accepted iteration commits

Start by reading Step 5a in workflows/crane.md and inspect the existing accepted-iteration commit prompt and summary fallback wording. Update the prompt to require the compact Changes: block before Run: {run_url}, then add the suggested prompt test to verify the subject template, ordering, and fallback guidance.

Background

In githubnext/apm, later Crane PR comments needed a reliable way to explain what a newly accepted commit actually did. Sometimes score fields, issue comments, or state-file summaries are unavailable, stale, or too sparse. The commit itself should carry enough structured information to be a fallback source.

APM updated its local Crane workflow prompt so accepted iteration commits include a compact Changes: section before the Run: URL.

Problem

Today an accepted Crane commit may only have a subject and a run URL. That is not enough for downstream summaries, PR comments, or issue updates to reconstruct the useful reviewer-facing facts.

When a PR has many iteration commits, reviewers need to know which module, milestone, or command family each commit advanced. A generic subject plus a workflow link forces everyone to open logs.

Proposed implementation

Update workflows/crane.md so Step 5a requires accepted iteration commit messages to use this shape:

[Crane: {migration-name}] Iteration <N>: <short description>

Changes:
- <compact bullet describing the main migrated unit or behavior change>
- <optional second bullet with test, parity, or milestone movement>

Run: {run_url}

Guidance:

• Keep Changes: compact and structured.
• Prefer one or two bullets, three max.
• Mention the migrated unit or behavior, not generic process.
• Mention test or parity movement when useful.
• Keep Run: as the final evidence link.

The accepted-iteration summary logic should then be allowed to fall back to the commit subject plus Changes: bullets when richer state is missing.

Suggested test coverage

Add a prompt test asserting:

• The commit subject template is present.
• Changes: appears before Run: {run_url}.
• The prompt says the Changes: block is used as summary fallback.

Acceptance criteria

• Accepted Crane commits contain structured Changes: bullets.
• Later PR or issue comments can summarize a commit without reading workflow logs.
• Existing commit subject and run-link behavior is preserved.
• The prompt test prevents regressions.

Provenance

This came from the githubnext/apm migration, where maintainer-facing summaries needed to describe what happened in each new Crane commit, not just that a commit was pushed.