docs(#1683): Diátaxis host-integration docs + versioning policy + ADR-1239 Accepted — Slice 3 (#1940)
- how-to: author a host-plugin (external-author guide against the SDK surface)
- tutorial: embed GSD in a new host (end-to-end programmatic-cli example)
- reference: the Host-Integration Interface (axes, adapters, handshake, profiles)
- explanation: interface versioning + deprecation policy (additive vs breaking,
PROTOCOL_VERSION bumps, deprecation window)
- ADR-1239 Status: Proposed → Accepted (Phases B–E shipped)