docs: plugin distribution design (ADR-0025/0026 + design doc) (#1428)

Design docs for distributing code-bearing marketplace contributions
(plugins with npm deps, drivers, connectors, AI extensions, client UI
plugins), alongside the existing metadata-only JSON package flow.

- ADR-0025: .osplugin artifact, build/sign/publish, permissioned install,
  trust/isolation tiers (in-process / QuickJS sandbox / out-of-process);
  bundled-default packaging; §3.10 scenario-review refinements; §3.11
  reconciliation with ADR-0019.
- ADR-0026: client-side UI plugin distribution (runtime:'ui' variant;
  U0 in-app module / U1 iframe sandbox; server-authoritative data).
- docs/design/plugin-distribution.md: supply-side companion design.

Docs only; no code or schema changes.

Author: os-zhuang