feat(FN-1469): create settings-loadable example plugin

- Add fusion-plugin-settings-demo package with settings schema (string, number, boolean, enum types)
- Add lifecycle hooks (onLoad, onTaskCreated, onTaskCompleted)
- Add tools (suggest_tags, status) with settings-driven behavior
- Add comprehensive tests with 24 test cases
- Add manifest.json for plugin metadata and README with Settings installation flow
- Update PLUGIN_AUTHORING.md with new example and install instructions
- Also includes: FN-1468 plugin wiring, FN-1456 abandon flow, FN-1429 always-green tests, FN-1440 Authentication in Settings, FN-1133 plugin hot-reload support

plugins/examples/fusion-plugin-settings-demo/vitest.config.ts

@@ -0,0 +1,8 @@
+import { defineConfig } from "vitest/config";
+
+export default defineConfig({
+  test: {
+    include: ["src/**/*.test.ts"],
+    pool: "threads",
+  },
+});