diff-explainlisted

# diff-explain ## Live context (when diffing a local branch) - Current branch diff stat: !`git diff $(git merge-base HEAD origin/main 2>/dev/null || git merge-base HEAD main 2>/dev/null || echo HEAD)...HEAD --stat 2>/dev/null || echo "no local diff vs main"` ## When to activate - User pastes a diff and asks "what does this do?" / "summarize" / "explain". - User shares a PR link or branch name and wants a quick read before reviewing. - Reviewing unfamiliar changes before merge. Do NOT activate when the user is asking for a detailed line-by-line review -- that's a different task. ## Steps 1. **Get the diff.** - PR link: `gh pr diff <url-or-number>` - Local branch: `git diff <base>...HEAD` (default base: `main` or `master`) - Pasted: use what the user pasted. 2. **Read it fully.** Don't skim the first 50 lines and guess. 3. **Group by concern.** Even if the diff touches many files, most diffs have 1-3 concerns (a bug fix, a rename, a new feature). Name each concern in plain language. 4. **Write ONE paragraph.** 3-5 sentences covering: - What changed (the concern, not the files). - Why it likely changed (if the commit message / PR title / linked issue says). - What risk it introduces (behavior change, API change, DB migration, dependency bump). 5. **Optional risk line.** If one specific thing deserves attention before merge, append one line starting with `Watch out for:`. ## Rules - No file-by-file walk. Group by concern, not by path. - No bullet lists.