fix: scope vitest to tests/ dir to avoid picking up stray worktree checkouts

Vitest's default glob isn't excluded for .claude/, so a sibling worktree
under .claude/worktrees/ (e.g. worktree-ui-basic) got silently collected
and run against this repo's current src/ via the "@" alias, doubling the
test count and running a stale test file against current code under a
shared identity. Found by an independent test-runner pass while verifying
the Assignment transfer fix.

vitest.config.ts

@@ -9,6 +9,16 @@ export default defineConfig({
   },
   test: {
     environment: "node",
+    // Scope explicitly to this repo's own tests/ dir — without this, Vitest's
+    // default glob also picks up any stray sibling git worktree checked out
+    // under .claude/worktrees/ (e.g. worktree-ui-basic), which isn't excluded
+    // by Vitest's defaults since those only skip node_modules/.git/etc, not
+    // .claude/. A stray worktree's stale test file still resolves the "@"
+    // alias to *this* repo's src/ (see resolve.alias above, which is
+    // __dirname-relative), so it silently runs old tests against current
+    // code under a shared identity — confirmed happening in practice once a
+    // second worktree existed.
+    include: ["tests/**/*.test.ts"],
     setupFiles: ["./tests/setup.ts"],
     // Each test file spins up its own throwaway Postgres database by
     // mutating process.env.DATABASE_URL in tests/setup.ts's beforeAll (see