Adds docs/adr/README.md as the in-repo copy of the ADR index (§70 of the
wiki ADR-Index page), including the "ADR-006 | Kysely contained inside DB
adapter" entry, the §46 six-sections mandate, the §71 architectural fitness
tests and the §68 architecture review gates. Committed ADRs in docs/adr/
must record a decision that matches their §70 row; the wiki page remains
the canonical index.
