Record the five 2026-10-04 decisions in the repo #55

Context

Five decisions were taken on 2026-10-04 without machine access:

1. Two-repo question: Path C (merge), fallback Path A on 2026-10-18 (see Confirm two-repo outcome (2026-10-18 deadline passed) #54).
2. Sovereignty wording: the stronger version from 2026-11-15, if the offline install ships.
3. README claims audit: 8 claim decisions.
4. README structural additions: Status, Known Limitations and For Reviewers sections, all greenlit.
5. SECURITY.md: narrow scope, email channel, standard SLA.

The 10-04 plan said a DECISIONS/ folder. The repo already uses docs/adr/ (ADR-0001), so decide which.

Done when

All five are committed through a PR in the chosen location.