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/cli/src/extension.ts

@@ -7,7 +7,7 @@ import {
   COLUMN_LABELS,
   type Column,
   type Task,
-} from "@kb/core";
+} from "@fusion/core";
 import { resolve, basename, extname } from "node:path";
 import { readFile } from "node:fs/promises";
 import { spawn, type ChildProcess } from "node:child_process";