test: enforce test-directory isolation across all packages

Introduce a shared test-utils module and global vitest setup that
guarantee tests never write to the real .fusion directory or leak temp
directories under /tmp.

Infrastructure:
- packages/core/src/__test-utils__/workspace.ts — tempWorkspace(),
  useIsolatedCwd(), trackForCleanup(), assertOutsideRealFusion() with
  auto-cleanup in afterEach.
- packages/core/src/__test-utils__/vitest-setup.ts — per-worker guard:
  chdirs each worker into an isolated tmp dir, wraps process.chdir to
  refuse the real .fusion, scopes tmp dirs under fusion-test-workers/
  (skips cwd change in thread-pool workers where chdir isn't supported).
- packages/core/src/__test-utils__/vitest-teardown.ts — globalSetup
  hook that wipes the shared parent even when workers are SIGKILLed.
- scripts/check-test-isolation.mjs + `test:isolated` / `test:check-
  isolation` scripts for CI.
- @fusion/test-utils alias + setupFiles + globalSetup wired into core,
  cli, engine, dashboard, tui vitest configs; matching tsconfig paths.

Test refactors (no behavior change):
- cli provider-settings, auth-paths, provider-auth — switch leaking
  mkdtempSync calls to tempWorkspace().
- core migration, first-run, store-backward-compat — replace manual
  process.chdir save/restore with useIsolatedCwd().
- tui fusion-context — replace 9 hardcoded tmp paths (collision-prone
  under parallelism) with tempWorkspace().
- dashboard useTheme, FileBrowser, TaskCard — resolve source-file reads
  against a PACKAGE_ROOT computed from import.meta.url instead of cwd,
  so tests don't depend on the process working directory.

Verified: full suite (~15,500 tests across 8 packages + plugins) passes
and the orphan-detector reports zero leaked temp directories after a
complete run.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/engine/tsconfig.json

@@ -3,7 +3,10 @@
   "compilerOptions": {
     "outDir": "dist",
     "rootDir": "src",
-    "types": ["node", "vitest/globals"]
+    "types": ["node", "vitest/globals"],
+    "paths": {
+      "@fusion/test-utils": ["../core/src/__test-utils__/workspace.ts"]
+    }
   },
   "include": ["src/**/*"]
 }

packages/engine/vitest.config.ts

@@ -10,10 +10,13 @@ export default defineConfig({
   resolve: {
     alias: {
       "@fusion/core": resolve(__dirname, "../core/src/index.ts"),
+      "@fusion/test-utils": resolve(__dirname, "../core/src/__test-utils__/workspace.ts"),
     },
   },
   test: {
     include: ["src/**/*.test.ts"],
+    setupFiles: [resolve(__dirname, "../core/src/__test-utils__/vitest-setup.ts")],
+    globalSetup: [resolve(__dirname, "../core/src/__test-utils__/vitest-teardown.ts")],
     maxWorkers,
     poolOptions: { threads: { minThreads: 1, maxThreads: maxWorkers }, forks: { minForks: 1, maxForks: maxWorkers } },
     fileParallelism: true,