Epic: Unified configuration #441

Spec: configuration.md (status)
Plans: phase-8/configuration.md ·
sops-config-discovery.md

One configuration model: a recursive tree resolved per profile, with two announcement paths, a per-key
overlay carrying a path-keyed set-by sidecar, and declared-type instantiation. The design is settled and
the code does not yet agree with it.

Also holds the pkg/devconfig versus cmd/internal/config overlap, which
step 55 raised and did not
settle.

Blocks: phase 3.3b of the Windows campaign — document.Write taking a root was deferred by ruling to
land with this work and the codec, because its remaining callers are ordinary config-shaped documents.