Move the complete research domain from docs/research to root-level research, update indexes and path-aware tooling, and align raw snapshot governance and CODEOWNERS.\n\nVerified with make test and GitHub CI.
- Add docs/concepts/keyword-system.md and wire it into README, docs indexes, taxonomy, llms.txt, and AI citation context.\n- Exclude unstable zread.ai links from lychee external link checks.