Add lightweight Fleetbase plugin runtime

[roncodes]

Summary

• add plugin manifest detection and skip plugin packages when mapping Ember engines
• add a constrained Universe plugin context for service lookup, menus, widgets, actions, hooks, events, and permission metadata
• wire plugin setup into the extension manager alongside existing engine extension setup

Validation

• PATH=/Users/ron/.nvm/versions/node/v22.22.2/bin:$PATH pnpm exec eslint addon/services/universe/plugin-context.js addon/services/universe/extension-manager.js addon/services/universe.js addon/utils/map-engines.js addon/utils/load-engines.js addon/utils/is-plugin-extension.js tests/unit/services/universe/plugin-context-test.js tests/unit/services/universe/extension-manager-test.js tests/unit/utils/map-engines-test.js tests/unit/utils/is-plugin-extension-test.js
• PATH=/Users/ron/.nvm/versions/node/v22.22.2/bin:$PATH pnpm build

Notes

• Targeted Ember browser tests built but failed before tests loaded because the existing test harness could not resolve @ember/string from Ember Data's vendor bundle.
• Documentation impact: introduces a new frontend plugin developer workflow and should be documented in fleetbase.io.
• API spec impact: none; plugins are frontend-only in v1.