fix(skills): move top-level origin frontmatter key under metadata

The official Agent Skills spec (agentskills.io/specification) whitelists exactly
6 top-level frontmatter keys (name/description/license/compatibility/metadata/
allowed-tools). A top-level `origin` key fails the official validator
(anthropics/skills quick_validate.py ALLOWED_PROPERTIES; skills-ref validate).

This moves `origin: X` -> `metadata.origin: X` across the canonical skills/
tree, preserving each value verbatim. Frontmatter-only, minimal diff.

- 251 SKILL.md updated (242 new metadata block, 9 appended to existing metadata)
- origin values preserved verbatim (verified 251/251)
- YAML validated on all changed files
- scoped to canonical skills/ only (docs/<lang> translations + tool mirrors
  .cursor/.kiro/.agents left untouched; presumably regenerated from canonical)

Addresses #2233

skills/springboot-patterns/SKILL.md

@@ -1,7 +1,8 @@
 ---
 name: springboot-patterns
 description: Spring Boot architecture patterns, REST API design, layered services, data access, caching, async processing, and logging. Use for Java Spring Boot backend work.
-origin: ECC
+metadata:
+  origin: ECC
 ---
 
 # Spring Boot Development Patterns