test(api): type-check harness and cover automigrate and auth isolation

- add tsconfig.tests.json and run tsc in test:api
- move vitest ProvidedContext types to provided-context.d.ts
- rename vitest.api.config.ts to .mts
- kill signal-test fixture with SIGTERM first so cleanup still runs
- add tests: no files in pb_migrations, independent user/guest clients
- docs: drop superuser env vars from tests in README, note tsc in testing.md

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EvmtStx737hu8gXe15e55W

tests/api/global-setup.ts

@@ -1,6 +1,5 @@
 import type { TestProject } from 'vitest/node';
 import { startPocketBase } from '../support/pocketbase-server';
-import '../support/clients';
 
 export default async function setup(project: TestProject) {
     const { url, dataDir, superuser, stop } = await startPocketBase();