Add complypack init command to generate complypack.yaml

[jpower432]

Summary

With the schema index (#30), users need to know the available platform names and write complypack.yaml by hand. A complypack init command would generate the config interactively or from flags.

Desired Behavior

# Interactive — prompts for platforms and sources
complypack init

# From flags — no prompts
complypack init \
  --schema kubernetes-deployment \
  --schema ci-github-actions \
  --source oci://ghcr.io/org/catalog:latest \
  --evaluator-id opa \
  --id my-pack \
  --version 0.1.0

Writes a valid complypack.yaml to the current directory. Lists available platforms from the schema index when run interactively.

Context

Follow-up from #30 (schema index). With 20 platform entries in the index, discoverability matters. The init command makes it easy to get started without reading docs.