feat(generator): support versioned_files on package definitions

[BryanFRD]

Closes #224

Adds an optional versioned_files list to packages, in the shape consumers already write. Entries with content are written relative to the repo root before the initial commit; format is informational. The default version.toml is unchanged, paths get the same traversal check as hooks, and the JSON schema and README are updated. Additive, existing definitions generate the same output.

Not done: warning on unknown keys under packages[] (the related idea in the issue).

[ferrfleet-interne]

No blocking findings. The write loop follows the existing hook pattern. Traversal is validated with the shared has_traversal. Files are written before chore: initial setup, so they land in that commit, and the integration test checks this. The schema, README and serde types agree.

Nit: a versioned_files entry whose path is version.toml (or <pkg.path>/version.toml) silently replaces the generated default, which contradicts "the default version.toml is still generated" in the README. Either document that it overrides the default, or reject that path in validate.rs.