Structured PR description auto-generation

[haasonsaas]

Problem

When a PR is opened, competitors auto-generate a structured PR description with:

• Title suggestion
• Summary of changes (bulleted)
• File-by-file walkthrough with change rationale
• Labels (bug, feature, refactor, etc.)
• Breaking change detection
• Testing notes

DiffScope has a --summary flag on the pr command, but it doesn't produce the structured walkthrough format that's become the #1 feature users evaluate when trialing AI code review tools.

Proposed Solution

Add a pr describe subcommand (or enhance pr --summary) that:

1. Reads the full PR diff (all commits, not just HEAD)
2. Generates a structured description:
   • Summary: 2-3 bullet points of what changed and why
   • Walkthrough: Per-file change descriptions with rationale
   • Labels: Auto-suggested labels (feature, bugfix, refactor, docs, test)
   • Breaking changes: Flagged if detected
   • Testing notes: What should be tested
3. Posts/updates the PR body on GitHub via the API
4. Optionally auto-runs on PR open via webhook

Competitive Context

• CodeRabbit: Full walkthrough with collapsible file sections, auto-labels
• Qodo Merge: /describe command generates structured PR body with labels and step-by-step walkthrough
• GitHub Copilot: Partial — generates summary but not full walkthrough
• Ellipsis: Auto-generates PR descriptions on open

Priority

Tier 1 — Table stakes. This is the feature new users evaluate first.