Spell out that SPDX tags must use native comments (`#` in code and TOML,
HTML comments in Markdown/Quarto) and call out formats like JSON that cannot
carry in-file blocks.
- Add an SPDX section with concrete examples and the strict-JSON caveat.
Explain next to the one-file workflow that message depth should follow patch
size—grouped bullets on large or multi-concern commits.
- Adds one bullet under coding rules pointing readers at `.github/prompts/commit.prompt.md`.
Replace outdated badges and roadmap prose with the current 2.0 focus areas,
installation notes, and pointers to AGENTS.md. Rewrite CONTRIBUTING as a
concise fork/test/layer-contract checklist aligned with the repo layout.
- README: drop stale Travis/Gitter stack; document legacy module caveat
- CONTRIBUTING: remove Quarto front matter; link architecture_layers checker
Jukka Aho