chore(test): split dashboard api/routes monoliths and document core fork-only constraint

Split packages/dashboard/app/__tests__/api.test.ts (~6.4k lines) into per-area files (auth, git, missions, projects, settings, tasks), and src/__tests__/routes.test.ts (~20k lines) into per-area files (agents, auth, automation, git, github, planning, settings, system, tasks, tasks-ops). The monolith files were dominating wall-clock for the dashboard suite under file-parallel execution.

Also document in packages/core/vitest.config.ts why the core suite cannot move to "threads": vitest-setup gates per-worker cwd on isMainThread (false in worker_threads, so isolation breaks), and setup-test-isolation writes process.env.HOME unconditionally (threads share env, so concurrent workers race).

Drop an unused execFileSync import from scripts/test-with-lock.mjs.

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

packages/core/vitest.config.ts

@@ -17,6 +17,17 @@ export default defineConfig({
       "./src/__test-utils__/vitest-setup.ts",
     ],
     globalSetup: ["./src/__test-utils__/vitest-teardown.ts"],
+    // Must stay "forks". Two thread-unsafe patterns block migration to "threads":
+    //
+    //   1. vitest-setup.ts:123 — `process.chdir(workerTempDir)` is gated by
+    //      `isMainThread`, which is `false` in worker_threads, so each thread
+    //      worker never gets its isolated cwd. Tests that rely on cwd being a
+    //      disposable temp dir would silently operate in the repo root.
+    //
+    //   2. setup-test-isolation.ts:15-16 — `process.env.HOME` is written
+    //      unconditionally in every setupFile invocation. Threads share
+    //      `process.env`, so concurrent workers race on HOME and the last writer
+    //      wins, breaking isolation for all other workers in the same run.
     pool: "forks",
     maxWorkers,
     poolOptions: { forks: { minForks: 1, maxForks: maxWorkers } },