feat(KB-330): rename internal packages from @kb/* to @fusion/*

- Rename @kb/core, @kb/dashboard, @kb/engine to @fusion/* namespace
- Update all import statements across 143+ files to use new package names
- Update workspace dependencies and root package.json references
- Fix bundler configurations (tsup, vite) for new package names
- Update test files and fix typecheck issues
- Add changeset file documenting the package rename

packages/engine/src/worktree-pool.test.ts

@@ -12,7 +12,7 @@ vi.mock("node:fs", () => ({
 import { WorktreePool, scanIdleWorktrees, cleanupOrphanedWorktrees } from "./worktree-pool.js";
 import { execSync } from "node:child_process";
 import { existsSync, readdirSync } from "node:fs";
-import type { Task, Column } from "@kb/core";
+import type { Task, Column } from "@fusion/core";
 
 const mockedExecSync = vi.mocked(execSync);
 const mockedExistsSync = vi.mocked(existsSync);