docs: add a pull request template, all of it optional

.github/PULL_REQUEST_TEMPLATE.md

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+<!--
+Nothing below is required. These are the sections the merged pull requests here
+tend to have, offered so you don't have to reverse-engineer them. Delete any
+that don't apply, rename them to say what you actually found, and write prose
+rather than filling in fields.
+-->
+
+## What this is
+
+What changes, and `Closes #123` if there's an issue. If someone reported it,
+say who and where.
+
+## Mechanism
+
+Why the old behaviour happened — the specific line, default or assumption
+responsible — rather than what you did about it. If you measured something,
+paste the numbers.
+
+## Scope
+
+What you deliberately left alone, and why. Anything you noticed while working
+and chose not to fix here belongs in this section too.
+
+## Tests
+
+What you added or changed. For a fix: revert the fix, keep the test, and say
+whether it goes red. A test that passes either way isn't testing the fix.
+
+## Verification
+
+The commands you ran and what they said — the ones CI runs are:
+
+```
+npm audit
+npm run check
+npm test
+cargo test        # in src-tauri/
+```
+
+And what you *didn't* verify: platforms you couldn't try, paths you reasoned
+about rather than ran. That's more useful to a reviewer than a list with no
+gaps in it.