Records the grill-with-docs decision on issue #88: replace the
hand-authored plugin/marketplace manifest model with Microsoft APM
(apm.yml + .apm/) as this repo's authoring source of truth. The
plugins/<name>/ monorepo-hybrid layout survives; marketplace.json and
provider plugin.json files become compiled output. Supersedes
ADR-0001; touches but does not resolve ADR-0006/0010/0014. Follow-up
work tracked in issues #89 and #90.