feat(KB-334): change default task prefix from KB to FN

- Change default taskPrefix from KB to FN and branch naming from kb/ to fusion/
- Update all test expectations and patterns for new naming convention
- Update settings UI placeholder text to reflect FN prefix
- Update dashboard, engine, and CLI components for fusion/ branch paths
- Add changeset documenting the breaking change for users

packages/core/src/db.test.ts

@@ -1,5 +1,6 @@
 import { describe, it, expect, beforeEach, afterEach } from "vitest";
 import { Database, createDatabase, toJson, toJsonNullable, fromJson } from "./db.js";
+import { DEFAULT_PROJECT_SETTINGS } from "./types.js";
 import { mkdtempSync, existsSync } from "node:fs";
 import { join } from "node:path";
 import { tmpdir } from "node:os";
@@ -99,7 +100,7 @@ describe("Database", () => {
       expect(row).toBeDefined();
       expect(row.nextId).toBe(1);
       expect(row.nextWorkflowStepId).toBe(1);
-      expect(row.settings).toBe("{}");
+      expect(row.settings).toBe(JSON.stringify(DEFAULT_PROJECT_SETTINGS));
       expect(row.workflowSteps).toBe("[]");
       expect(row.updatedAt).toBeTruthy();
       // updatedAt should be a valid ISO timestamp