The scaffold-repo skill emits an environments block Terraform accepts

## Summary

Update the two `environments = ["hackage"]` examples in
`.claude/skills/scaffold-repo/SKILL.md` to the map-of-object shape the
`github_repository` module actually takes.

## Motivation

#270 moved the module's `environments` variable from `set(string)` to
`map(object(...))`, but the skill still teaches the list-of-strings form:

- `.claude/skills/scaffold-repo/SKILL.md:47` — `published →
environments = ["hackage"]`
- `.claude/skills/scaffold-repo/SKILL.md:111` — `Environment secrets (e.g. the
Hackage token for environments = ["hackage"]) are injected out of band.`

A scaffold run that follows the skill emits a module block that fails
`terraform validate`, so the drift costs a round trip on every new published
repo. Found while working #212, which touches the same variable.

## Scope

- Rewrite both occurrences as `environments = { hackage = {} }`, matching
`terraform/alunduil/repositories.tf`.
- Check the surrounding sentences still read correctly against the current
variable — line 111 describes out-of-band secret injection, which the shape
change doesn't affect, but the phrasing wraps the example.

## Acceptance criteria

- [ ] No `environments = [` remains in `.claude/skills/scaffold-repo/`.
- [ ] The shape shown matches `terraform/modules/github_repository/variables.tf`
at HEAD, including the `reviewers` field added by #212.
- [ ] A block copied verbatim out of the skill passes `terraform validate`.

## Additional context

Sequenced after #212, which adds `reviewers` to the same variable — fixing this
first would document a shape that goes stale again on merge.