Require rich Crane PR update comments

[Copilot]

Description

Crane accepted-iteration PR updates currently identify only the pushed SHA, forcing reviewers to open the commit or diff to understand the new work. This updates the Crane workflow prompt so existing-PR updates carry the same concise review context as the PR body, migration issue, and repo-memory history.

• Shared summary source

  • Defines one accepted-iteration summary reused across PR body, PR comment, migration issue comment, and repo-memory history.
  • Includes iteration, milestone/focus, change summary, commit/run links, score/delta/progress/parity, and CI fix attempts when available.

• Mandatory PR update comment

  • Requires a separate add-comment safe output for accepted updates to existing PRs.
  • Prevents relying on the default push-to-pull-request-branch “Commit pushed” comment as the only reviewer-facing signal.

• Commit fallback

  • Strengthens commit guidance so accepted commits include compact Changes: bullets before Run:.
  • Lets later summaries fall back to commit content when score/progress fields are missing.

• Bounded output

  • Keeps comments short.
  • Ignores ci: trigger checks commits unless they are the only new commit.

Example target shape:

[bot] **Iteration {N} accepted** -- [Crane run]({run_url})

- **Commit**: [`{short_sha}`]({commit_url})
- **Change**: {one-line summary}
- **Milestone**: {milestone or current focus}
- **Score**: {migration_score} (best: {best_metric}, delta: {+/-delta})
- **Progress**: {progress fraction}
- **Tests/parity**: {parity_passing}/{parity_total} passing

Type of change

• Bug fix
• New feature
• Documentation
• Maintenance / refactor

Testing

• Tested locally
• All existing tests pass
• Added tests for new functionality (if applicable)