improvement: extend check:claude-plugin validator for missing plugin components

[Luis85]

Context

Part of the plugin audit in #444 (§1, R8). The current check:claude-plugin.ts validator does not check for several components that are required for a complete, adopter-ready plugin package. These gaps mean the build can succeed while the plugin is missing critical functionality.

Current gaps in the validator

Component	Currently checked	Risk
hooks/hooks.json	✗	Adopters miss enforcement hooks
monitors/monitors.json	✗	Operational bots not discoverable
settings.json (default agent)	✗	No default agent experience
.mcp.json	✗	No MCP config home
Skill description field quality	✗	Skills invisible to runtime
Hook migration status	✗	Hooks left in settings-only mode
$schema in plugin.json	✗	No manifest validation
category in marketplace.json	✗	Breaks marketplace display

Work

Extend scripts/check-claude-plugin.ts to:

1. Assert hooks/hooks.json exists in plugin package
2. Assert settings.json exists in plugin package
3. Assert plugin.json has $schema field
4. Assert marketplace.json plugin entry has category field
5. Warn (non-fatal) if any SKILL.md in the plugin package has no description: in frontmatter
6. Warn (non-fatal) if monitors/monitors.json is absent

Add claude plugin validate as a CI step alongside the custom check (if available in the installed Claude Code version).

Definition of Done

• All 4 hard assertions pass in CI
• Skill description warning fires for any skill missing description:
• monitors/monitors.json absence produces a warning, not a failure (experimental component)
• npm run check:claude-plugin reflects new checks
• Verify gate green

References

• Parent audit: research: Claude plugin developer best-practices audit — gaps, standards, refactor opportunities #444 §1, R8
• Official validate command: https://code.claude.com/docs/en/plugins