Add skill author scaffolding and one-command validation

[Scriptception]

Problem

Author documentation is strong, but creating a skill still requires manual directory, metadata, structure, reference, test, and eval-fixture setup that is easy to make inconsistent.

User outcome

An author can create a structurally complete skill and receive actionable local feedback before opening a pull request.

Scope

• Add an init command for canonical metadata and skill structure.
• Add a validate command covering metadata, references, adapter rendering, structure, catalog output, and eval fixture presence.
• Offer templates without inventing domain guidance.
• Document the review path for official and organization-specific skills.

Non-goals

• Generating substantive security advice with an LLM.
• Automatically accepting a skill into the official catalog.

Acceptance criteria

• A generated skill passes all structural checks before content is added.
• Validation names the exact file, rule, and remediation for each failure.
• The command works without network access for local checks.
• Author docs use the command as the primary contribution path.

Validation

• Generate, complete, validate, render, install, and remove a fixture skill.
• Test malformed metadata, missing references, unsupported agents, and stale generated output.

Relationships

• Uses the machine-readable catalog schema.
• Feeds verifiable evaluation work.