Decouple maturity: experimental from physical asset location

[WilliamBerryiii]

Summary

Decouple maturity: experimental from filesystem placement for experimental artifacts.

Today the manifest requires experimental artifacts to physically live under experimental/ subtrees (for example .github/skills/experimental/, .github/agents/experimental/). During review of #2048 it became clear that the physical asset location should not be a requirement for marking an artifact as experimental. The maturity: field in collections/core-manifest.yml is the single source of truth for maturity; an artifact's directory should not have to encode it.

Proposed change

• Remove the physical-location requirement so an artifact can carry maturity: experimental regardless of where it lives on disk.
• Update any validation/projection logic that infers or enforces maturity from the experimental/ path segment so it relies on the manifest maturity: field instead.
• Update docs/conventions that state experimental artifacts must live under an experimental/ directory.

Context

• Raised in code review of Consolidate collection manifests and remove attributions #2048 (manifest consolidation / attribution removal).
• Source of truth: collections/core-manifest.yml (maturity: per artifact).

Acceptance criteria

• An artifact can be maturity: experimental without residing under an experimental/ directory.
• Validation no longer couples maturity to filesystem location.
• Conventions/docs updated to reflect the decoupling.