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/merger.ts

@@ -1,6 +1,6 @@
 import { execSync } from "node:child_process";
 import { existsSync, readFileSync } from "node:fs";
-import type { TaskStore, Task, MergeResult } from "@kb/core";
+import type { TaskStore, Task, MergeResult } from "@fusion/core";
 import { createKbAgent } from "./pi.js";
 import type { WorktreePool } from "./worktree-pool.js";
 import { AgentLogger } from "./agent-logger.js";