refactor: the shortname is `helm`, not `hath`

`hath` was an AI-introduced abbreviation for Human at the Helm. Nobody chose it, and it had
reached a skill directory, the manifest target and the README before anyone noticed. It also
reads as archaic English for "has", which in a Bible-software context looks like a deliberate
styling rather than an acronym.

- skills/hath-check -> skills/helm-check, including its `name:` frontmatter
- the manifest target docs/ai-context/hath-manifest.yaml -> helm-manifest.yaml
- README.md and .claude/skills/install/SKILL.md updated to name /helm-check

Migration note for already-installed projects: the manifest entry is create-or-replace, so
re-running /install writes helm-manifest.yaml and leaves the old hath-manifest.yaml orphaned
beside it. /helm-check reads the manifest by name, so until a re-install those projects report
a missing manifest and /hath-check no longer exists as a command.

The clean break is deliberate rather than a dual-name transition: Scripture Pipelines rule 18
`one-design` requires a design change to be completed in one pass, and an older path that must
survive to name who depends on it and when it ends.

Guarded from the sibling repository by tests/test_shortname_is_helm.py, which checks this clone
when $HELM_REPO points at it and skips where no clone exists.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
