Add plugin package layout smoke test

[baskduf]

Description

Add a focused smoke test for the plugin package layout from a Codex marketplace/install perspective. This should catch missing files and broken manifest references before release.

Acceptance Criteria

• Validate .codex-plugin/plugin.json required fields.
• Validate the manifest skills directory exists and resolves inside the plugin root.
• Validate SKILL.md exists for the plugin name.
• Validate bin wrappers target existing scripts.
• Validate scripts imported by CLI entrypoints are tracked and present.
• The test should catch a missing shared module such as codex_fable_state.py.

Notes

This complements the current manifest/path tests and wrapper smoke tests.