Branch briefs

A branch brief is a Markdown or JSON artifact that describes what changed on a branch.

It contains:

• Repository, branch, and base details.
• Current working tree status.
• Commits since base.
• Changed files.
• Diff stats.
• Risk level and risk notes.
• Suggested verification.
• Rollback notes.
• Human decision needed.
• Optional Copilot review context.

The artifact is meant for pull request descriptions, Actions job summaries, review handoffs, release notes, and agent completion reports.