github-pull-request-describe

name	github-pull-request-describe
description	Generate a PR title and description from the committed diff against the default branch. Use when the user asks to draft, write, or prepare a pull request description/body. Does not create the PR.

Pull Request Description

Generate a PR title and description from the diff against the default branch.

Task

1. Detect the default branch (git symbolic-ref refs/remotes/origin/HEAD | sed 's@^refs/remotes/origin/@@', fallback main)
2. Run git fetch origin, then git log --oneline origin/<default>..HEAD and git diff origin/<default>...HEAD
3. Draft a PR title and description from committed changes only (ignore working tree)

Rules

• Title: under 72 characters, imperative mood
• Why: the problem / motivation / existing constraints that made this work necessary (1-3 sentences). Describes the situation before the fix, not the fix itself.
• What: the design decision taken and its reasoning — alternatives considered, chosen approach, rejected options and why (aim for 2-6 sentences of prose, or 2-5 bullets). Do NOT restate facts visible from the diff (changed file names, added function names, line counts, API signatures). Reviewers will read the diff for implementation detail; they need the design thinking from you.
• References: linked issues/PRs as bullets (- #123), or the single line N/A when there are none.
• If $ARGUMENTS is provided, use it as additional context.

Boundary rule for Why vs What: "why is this work needed" belongs in Why; "what was decided, among which alternatives, and why that choice" belongs in What.

Output

Do NOT create the PR. Write the draft to /tmp/pr-description.md as the canonical artifact — chat rendering can introduce trailing whitespace invisibly, so the file (not the chat output) is the source of truth. Verify cleanliness and copy to the clipboard:

grep -nE '[[:blank:]]+$' /tmp/pr-description.md && echo "FAIL: trailing whitespace" || pbcopy < /tmp/pr-description.md

Also show the same content in a single fenced code block for in-chat preview (any fence style — triple backticks or tildes — works; the template below uses ~~~). Do not leave trailing whitespace on any line.

<title>

## Why
<motivation — 1-3 sentences>

## What
<design approach and reasoning — prose or bullets, not a diff summary>

## References
- <issue/PR>

When there are no references, replace the bullet under ## References with a single bare line N/A (do NOT write - N/A).