bregctl init writes JSON into files named .yaml

[jeremi]

Problem

bregctl init writes registry.yaml and tests/journeys.yaml as JSON documents, byte for byte,
with a .yaml extension. Hand-authored projects in the same fixture set (the facility fixture's
registry.yaml, Casework's casework.yaml, module files) are real YAML, so the inconsistency is
in the generator rather than in what the loader accepts.

Cost

registry.yaml is the first file an adopter opens after bregctl init, and the file they are
expected to edit by hand.

1. No comments. JSON has no comment syntax, so a new project cannot record why a field is set
   the way it is, in the one file most likely to need that.
2. Editor tooling misreads it. Tools key off the extension and offer YAML behaviour over JSON
   content.
3. It teaches the dialect. Everyone starting from the template inherits JSON style, so real
   projects drift away from the documented YAML examples.

Request

Have init emit actual YAML, ideally with comments on the fields a new project is expected to
change.

Found against origin/main at ec0573f3d, binaries 0.32.0-dev.