Follow-up to the plugins framework (#3041): wires the plugin hooks end-to-end and adds full test coverage.
- Wires `onRequest` / `onResponse` / `onError` hooks into the chat pipeline (`chatCore` now imports from the unified `hooks` registry).
- Loads active plugins on server startup (`pluginManager.loadAll()` in `server-init`) so they survive restarts.
- Ships a `welcome-banner` example plugin (`examples/plugins/`) + test fixture.
- Adds a comprehensive plugin test suite (manifest, db, config, hooks, manager lifecycle, loader IPC, permissions, scanner, welcome-banner e2e).
Integration fixes applied during review:
- `manager.install` now removes an orphaned `destDir` (DB row gone but files left on disk) before the atomic rename, guarded by path containment — it previously failed with `ENOTEMPTY` (a regression surfaced by the new lifecycle test).
- `plugins-db` / `plugins-manager-lifecycle` tests now initialize the DB via the real migration `076` (`getDbInstance`) rather than relying on ambient state, so a missing/renumbered migration fails loudly instead of being masked.
348/348 plugin tests pass; typecheck / cycles clean.
Co-authored-by: oyi77 <14921983+oyi77@users.noreply.github.com>
