Repository collaborators are declared in Terraform and stale write access is removed

## Summary

Manage direct collaborators in the `github_repository` module, then remove the write-access accounts on `zfs-replicate` that have never contributed.

## Motivation

`zfs-replicate` has twelve direct collaborators with write access. One of them, `perniferuse`, has two commits; the other eleven have none:

```
moiseev Flameeyes devx velveteer albmin noellesage
matt-kempster jjalfaro9 eazeye ahajicek07 jess98
```

Write access is enough to merge a pull request. The only thing standing between those accounts and an unreviewed merge today is the `require_code_owner_review` rule on the default branch, which #444 is about to exempt Renovate from.

The module declares no collaborator resource, so none of this is visible to `terraform plan` and none of it drifts back. The lists came from `GET /repos/alunduil/zfs-replicate/collaborators` and `GET /repos/alunduil/zfs-replicate/contributors` on 2026-08-25.

## Scope

- Add `github_repository_collaborators` to `terraform/modules/github_repository`, with a variable for the set and an empty default. That resource manages the whole list, so an undeclared collaborator is removed on apply.
- Check every other managed repository for the same drift before applying, and record what is found.
- Decide whether `perniferuse` keeps access.

## Out of scope

The ruleset and automerge changes in #444. Organisation-level or team-level access, of which there is none here.

## Acceptance criteria

- [ ] The module declares collaborators, and a collaborator added outside Terraform is removed on the next apply
- [ ] The eleven zero-commit write accounts no longer have access to `zfs-replicate`
- [ ] The decision on `perniferuse` is recorded here
- [ ] Every other managed repository is checked for the same drift and the findings recorded
- [ ] `terraform plan` is clean after the apply

## Additional context

- #444 exempts Renovate from the review rule that these accounts are otherwise gated by, which is what surfaced them.