Evaluate hlint `functions` bans for additional partial functions

## Summary

Consider extending the hlint `functions` restriction beyond `unsafePerformIO`
to hard-ban selected partial functions.

## Motivation

Raised while assessing the config in #132. The config already uses
`- functions: [{name: unsafePerformIO, within: []}]`. Candidates for the same
treatment: `error`, `head`, `fromJust`, `undefined`. Current lean is mostly
redundant — the enabled `partial`/`partial-strict` groups already flag these,
and the tests use `fromJust` deliberately (covered by the existing
`Avoid partial function` ignore for `Data.CollectionJSONSpec`). Filed to
capture the evaluation.

## Scope

- Identify any partial function not already covered by the partial groups that warrants a hard `within: []` ban (stronger than a group warning).
- Decide enable vs skip.

## Acceptance criteria

- [ ] Decision recorded; if adopted, `functions` list extended and CI green.