Evaluate hlint `generalise` group (`map`->`fmap`, `++`->`<>`)

## Summary

Decide whether to enable the hlint `generalise` group as a modernization
guard.

## Motivation

Surveyed while assessing the config in #132: zero hits on current `src/`/`test/`.
The group generalises `map`->`fmap`, `++`->`<>`, `x /= []`->`not (null x)`,
`maybe []`->`foldMap`. Unlike `partial-strict` (which reinforces the repo's
existing anti-partial stance), this is a pure style opinion with no existing
config precedent to lean on — so it warrants a deliberate call rather than a
silent enable.

## Scope

- Decide enable vs skip.
- If enabled, add `- group: {name: generalise, enabled: true}` to `hlint.yaml`.

## Acceptance criteria

- [ ] Decision recorded with rationale.
- [ ] If enabled, `hlint.yaml` updated and CI green.