Enable hlint `Use explicit module export list` warning

## Summary

Turn on hlint's off-by-default `Use explicit module export list` hint to
guard PVP-classified export surface.

## Motivation

Under PVP an incidental export forces a major version bump (see the scope
discipline section in `CLAUDE.md`). This hint bans `module X (module X) where`
and implicit whole-namespace modules, so a future module can't silently
export its internals. Surfaced while assessing the full hlint config in #132.
Zero hits today — the sole module already carries an explicit export list —
so it's pure forward insurance.

## Scope

- Add `- warn: {name: Use explicit module export list}` to `hlint.yaml`.

## Acceptance criteria

- [ ] Hint enabled in `hlint.yaml`.
- [ ] CI stays green.