plan(019): plugin system plan + research + data-model + contracts + quickstart
Phase 0 research resolves all 12 open decisions (interface shape, registration,
host API, dynamic toggle, migrations, UI panel integration, config format,
testing, boundary enforcement, squash, failure notification, integration test).
Phase 1 artifacts: data-model.md (Plugin, Registry, Migration, Host, Status,
Backup), contracts/plugin.md (Plugin + HasX interfaces), contracts/host.md
(Host struct + plugintest constructor), contracts/rest.md (/api/plugins/*),
quickstart.md (end-to-end "hello" plugin in 8 steps).
All ten constitution gates pass.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>