docs(readme): accurate description, validate command, DLA links (stacked on #76)

[simongonzalezdc]

README audit fixes (docs-only). Stacked on #76 (base branch fix/devarch-0.4.1-readiness) so it doesn't conflict with #76's README edits; GitHub will retarget this to main once #76 merges.

• Wrong product description fixed: TL;DR and the generated "What is DevArch Framework?" block called it a "development architecture framework and method". Rewritten from the README's own intro (local git repository archaeology).
• devarch validate described as "Validate project configuration"; it validates the generated history HTML (matches the CLI help and Prepare DevArch 0.4.1: bind derived evidence and disable unsafe legacy paths #76's readiness note).
• Removed the duplicated Dev Learning Archaeologist callout; updated its links to simongonzalezdc/dev-learning-archaeologist (the old KyaniteLabs URL only redirects).
• Forgejo link labeled "(private, maintainers only)"; issue/PR guidance points to GitHub.

License (Apache-2.0, approved by Simon): already handled in #76 (README badge + License section + setup.py license/classifier → Apache-2.0, matching LICENSE). There's no pyproject.toml. No further license change is needed in this PR.