bregctl init: make the example project exportable to an Evidence source unmodified

[jeremi]

bregctl init's example project (the template in crates/registry-bregctl/src/lib.rs) and the checked-in tutorial project under products/breg/evidence/registry/ disagree in three places that matter once a project is exported with generate evidence-source.

1. The template's code field is {type: string, required: true, maxLength: 64} with no minLength, while the tutorial project declares minLength: 1. The exporter refuses a selector profile on a field that accepts the empty value, so a project that adds a profile on the template's code is refused with the fix named. Declare minLength: 1 in the template.
2. The template ships no selectorProfiles and no lookup grant, so it cannot feed the exporter unmodified; the live lifecycle test authors both. If a fresh project is meant to be exportable, the template is the place.
3. The starter seeds (clients.yaml) use registrationNumber while the journeys (tests/journeys.yaml) use registration-number, and the alias is not explained anywhere a reader would look.

Origin: review of PR #865.