test(skills): cover plugin namespace loading

[anp-oai]

Why

Before changing plugin namespace loading performance, lock down the existing behavior so the same cases can be validated before and after the optimization.

What

• cover mixed plain and nested plugin skills under one scan root
• cover inherited, nested, and invalid-manifest namespace precedence
• cover symlinked plugin skill directories, symlinked plain directories, and scan-root ancestor symlinks

Validation

• just test -p codex-core-skills namespace on unmodified main: 12 passed