Create GitHub issues from a PR's work, and update the PR

Takes work that's already happened on a branch (or that the user is about to describe) and turns it into GitHub issues — one flat issue for a single atomic change, or a parent epic plus child issues when the work genuinely splits into distinct pieces — then rewrites the pull request's title and description so the description carries an itemized, correctly nested list of every issue it's tracked by.

The two halves of this (creating issues, updating the PR) are meant to happen together: an issue list in a PR description is only useful if the numbers in it are real and current, and issues filed for a PR are only useful if the PR actually points back at them.

Workflow