Revisit hlint `extra` group if the `extra` dependency is adopted

## Summary

Track re-evaluating the hlint `extra` hint group, which was surveyed and
left off in #132.

## Motivation

#132 ran the `extra` group against `src/` and `test/`: zero hits, and every
hint rewrites toward helpers in the `extra` package (`whenJust`, `concatForM`,
`maybeM`, `eitherM`, …) that this library doesn't depend on. Group-enabling it
would nudge contributors to add a dependency for stylistic sugar, so it was
declined. The calculus flips only if `extra` ever becomes a real dependency —
this issue captures that trigger so the decision isn't lost.

## Scope

- If/when `extra` becomes a dependency, enable `- group: {name: extra, enabled: true}`, survey hits, and fix/ignore per hint.
- Otherwise close as won't-do once it's clear the dependency won't be taken on.

## Acceptance criteria

- [ ] Decision recorded: enable, cherry-pick specific hints, or close.

## Additional context

Refs #132.

Start by reading #132 and checking whether the project has adopted the extra dependency and where its HLint groups are configured. If the dependency is present, run the extra group against src/ and test/, then record whether to enable it, select specific hints, or close the issue; otherwise document the decision to close it as won't-do.

索引モデルが issue の本文から書いたものです。