feat(schemas): add Talos image build schema

[jmgilman]

Summary

• add a Talos ImageBuild schema for manual bootstrap-cluster image builds
• generate Go types and embedded schema source for the new talos package
• register schemas/lab in the Release Please manifest so the next schema release can be cut
• serialize generated schema checks before schema and labctl Go tasks to avoid generated-file races

Validation

• moon run schemas:check --summary minimal
• moon run labctl:check --summary minimal
• moon ci --summary minimal
• cue eval of a minimal talos.#ImageBuild config