docs: honest upstream attribution + base_model, drop euphemism

Add LICENSE and NOTICE files for license compliance

Add NOTICE: Copyright Zen Authors

Brand: remove upstream model references

fix(readme): frontmatter:tags+=['zen-5']

weights: server-side copy from Qwen/Qwen3-Coder-Next

readme: strip upstream brand names (Zen-only branding)

readme: clean Zen5 branding (weights staged)

initial commit