Harden gate scripts (schema validation, security scan, size audit) and document failure modes.

[kayodebristol]

From ROADMAP Phase 1 (docs/ROADMAP.md).

Item:

• Harden gate scripts (schema validation, security scan, size audit) and document failure modes.

Context:

Phase 1 — Registry Maturity

• Publish contributor guide + submission PR template.
• Harden gate scripts (schema validation, security scan, size audit) and document failure modes.
• Add compatibility checks for Radix and design‑dojo versions.
• Establish versioning policy and deprecation workflow for plugins.