docs(adr): ADR-0027 metadata-authoring — staged authoring, publish & promotion lifecycle (#1427)

Adds ADR-0027 proposing @objectstack/metadata-authoring, a transport-agnostic
package owning the metadata authoring lifecycle: stage (draft, no DDL) → diff/
validate → preview → publish (seal + batched DDL + pointer-swap activation) →
promote Dev/Staging/Prod → rollback/deprecate. Grounded in the existing data
model (sys_package / sys_package_version / sys_package_installation / sys_metadata),
with a mainstream-platform comparison, long-term north-star capability map, and a
value-front-loaded M0–M5 roadmap. Boundary cross-ref to ADR-0025/0026 (distribution).

Author: os-zhuang