[Build/CI] Add read-only plugin and skill validators

[ashleyshaw]

Build/CI Summary

Add read-only validators for plugin manifests and skill folder shape.

Parent epic: #285.

Steps / Checklist

• Define manifest validation rules.
• Define skill folder validation rules.
• Validate SKILL.md presence and naming.
• Validate manifest references resolve to files inside the plugin package.
• Add npm run validate:plugins.
• Add npm run validate:skills.
• PR uses correct branch prefix ci/ or build/.

Acceptance Criteria

• Invalid plugin manifest fails validation.
• Missing skill SKILL.md fails validation.
• Missing referenced plugin file fails validation.
• Commands do not modify files.
• Documentation/changelog updated.

[linear-code]

LS-629