Decouple maturity: experimental from physical asset location

## 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 #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.