weft v0.3.7 — the model accessors thread needs

Two additive accessors (ADR 0006 amendment), released first so
weft/thread v0.2.0 can require a root that actually provides them —
the two-phase tag process introduced at v0.3.2.

• (*Agent).Model() — the model as the loop calls it,
  WrapModel middleware included, in the installed order. The
  session layer summarizes with the session agent's own model by
  default (ADR 0020 §2).
• The Unwrap() Model convention on model middleware wrappers,
  beside Info()/InfoOf: mw.Retry, mw.Fallback (its primary),
  mw.Log and mw.RepairJSON implement it; weft.Unwrap(m) walks
  one level and returns nil on a non-wrapper.

No behaviour change; apidiff clean (additive) against v0.3.6.