feat(plugins): fold first-party plugins in-tree; bundled + external roots

First-party plugins (host_agent, http, snmp, traefik, unifi, docker) are now
tracked under plugins/ and baked into the image, so they version atomically
with core — ending the cross-repo import drift the roundtable->steward rename
exposed. History for these files is preserved in the archived Roundtable-plugins
repo.

Plugin discovery becomes multi-root: PLUGIN_DIR (single) -> PLUGIN_DIRS
(bundled first, then external) + PLUGIN_INSTALL_DIR. Bundled ships in the image;
third-party plugins still mount at runtime into the external root
(STEWARD_PLUGIN_DIR, default /data/plugins) and downloads/installs land there.
Bundled shadows external on a name collision.

- config.py: load_bootstrap returns plugin_dirs + plugin_install_dir
- app.py: iterate PLUGIN_DIRS at the migration + load sites
- migration_runner.py: discover_all_in() unions every plugin root
- plugin_manager.py: resolve_plugin_path() (pure, first-root-wins); load /
  install / hot-reload span all roots; installs target the external root
- settings/routes.py: _discover_plugins scans all roots, dedup bundled-first
- Dockerfile: COPY plugins/ ; docker-compose: drop host bind, document external
- tests/test_plugin_dirs.py: resolution, multi-root discovery, bootstrap split

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -6,8 +6,10 @@ services:
     ports:
       - "5000:5000"
     volumes:
+      # First-party plugins are baked into the image at /app/plugins.
+      # Third-party plugins persist in the external root /data/plugins
+      # (inside app_data); STEWARD_PLUGIN_DIR defaults there, no extra mount needed.
       - app_data:/data
-      - ./plugins:/app/plugins:ro
       - /mnt/Data/traefik/log:/var/log/traefik:ro
       - /var/run/docker.sock:/var/run/docker.sock:ro
     environment: