Reject top-level version in package validation

[Ftgn-dpA]

Problem

AGENTS.md requires the skill version to live under metadata.version and forbids a top-level version. This layout was adopted after #41 because version is not a supported top-level Agent Skills frontmatter field.

The package validator currently rejects two disallowed top-level fields but omits version. Adding a duplicate top-level version therefore still reports the package as valid.

Change

Add version: to the existing unsupported-field check. The check is anchored at the start of a line, so the valid indented metadata.version remains allowed.

Verification

• Reproduced on main: a duplicate top-level version exits successfully.
• With this change, the same fixture exits with Remove unsupported YAML field: version.
• python scripts/validate-package.py
• npx --yes skills@1.5.20 add . --list
• claude plugin validate .
• git diff --check

Developed with AI assistance (Codex).