caseworkctl init --template should publish its possible values

[jeremi]

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.