Custom Vale rules are covered by a regression fixture

## Summary

The repo's custom Vale rules are hand-verified once at authoring time and never again. A fixture asserting each rule's expected hits would catch a regression when a pattern is edited.

## Motivation

`Custom.DateFormat` (#291) was validated against 18 cases — 5 it must flag, 13 near-misses it must not, including "The team may 3 times revisit that decision", "In June 2026", and a `simonwillison.net/2026/May/25/` URL. Several of those encode real reasoning about why the pattern is shaped the way it is: the `Month YYYY` cases are the reason the month alternation is spelled out twice rather than deduplicated.

None of that survived the PR. The corpus was a temp file, so the next person to edit the regex re-derives the constraints or silently breaks one.

`Custom.Spelling` has the same exposure, though its surface is a vendored dictionary rather than an authored pattern.

## Scope

- A fixture per custom rule holding lines that must flag and lines that must not.
- A runner asserting Vale reports exactly the expected hits — the "must not" half is the valuable half, and a plain Vale run can't express it.
- Wire into `.pre-commit-config.yaml` and therefore CI.

The fixture can't simply be linted by the existing `vale` hook: it contains intentional violations, so it would fail the blocking run. It needs a runner that inverts the assertion, and a path excluded from the normal hook.

## Acceptance criteria

- [ ] Editing a custom rule so it stops flagging a known-bad line fails the suite.
- [ ] Editing a custom rule so it starts flagging a known-good line fails the suite.
- [ ] `pre-commit run --all-files` passes on an unmodified tree.

## Additional context

This is the repo's first test infrastructure — there is currently no test directory, no `test` script in `package.json`, and no fixture precedent. `scripts/install-lychee.sh` plus the shellcheck/shfmt hooks are the closest existing pattern, so a shell runner under `scripts/` would fit the grain. Worth weighing that new machinery against a personal blog's two custom rules before building it.