Module Aggregator/Clean/Pipeline

Description

Compose the per-step cleaners into the public cleanArtifact entry point.

Applied transformations (in order):

1. Strip leading YAML front-matter (---\n…\n---\n).
2. Strip per-artifact SPDX tags.
3. Strip ISMS/owner/classification banners (emoji rows, shields.io badges, <p align="center"> blocks, and the separator --- that usually follows them).
4. Remove the artifact's own H1 and demote every other heading by one level so the aggregate has a single H1.
5. Strip the operational metadata preamble (**Run:**, **Window:**, …).
6. Rewrite repo-relative links/images to absolute GitHub URLs.
7. Deduplicate mermaid fence bodies on a per-document basis.
8. Collapse runs of 3+ newlines and append a final \n.