caseworkctl init --template should publish its possible values

## Problem

`caseworkctl init --template` accepts a free-form string and lists no possible
values in its generated command metadata. Invalid values are rejected only after
argument parsing. This differs from the closed template catalog now published by
`bregctl init --template`.

## Expected

- Publish the maintained Casework template ids as possible values in Clap and the
generated CLI reference.
- Keep template selection backed by the owning starter catalog rather than a
duplicated list.
- Add command-metadata and invalid-value regression coverage.

Found while reviewing #1016. The behavior predates that pull request.

Start at the caseworkctl init --template command definition and its owning starter catalog, then compare the published metadata with bregctl init --template. Add command-metadata and invalid-value regression coverage. Done means maintained Casework template ids appear as Clap and generated-reference possible values while selection still uses the owning catalog.

Written by the indexing model from the issue text.