Pull request descriptions

A PR description is a request for someone's attention. You are asking a colleague to spend twenty minutes finding your mistakes — the description's job is to make those twenty minutes land where they are most useful.

The most common failure is describing what changed, which the diff already shows, while omitting what to worry about, which only you know.

1. Open with what and why, in two sentences

The reader should know within one screen whether this concerns them.

Fixes scheduled jobs running an hour early during BST by removing a double
timezone conversion. Affects anything reading `jobs.run_at` — the value is now
always UTC.