test: isolate quick-start case fixtures

[Lint111]

Summary

• Give the quick-start API suite a unique temporary HOME.
• Import WebServer only after that environment is installed, so its module-level case directory resolves inside the fixture.
• Restore the caller's HOME and delete only the temporary fixture directory during teardown.

Root cause

The suite previously used the real ~/codeman-cases directory. Its cleanup recursively removed the default testcase case, which can be the checkout containing the test runner itself. In that layout, running CI deleted the repository, Git metadata, and dependencies while Vitest was still executing.

Scope

This is the self-contained test-fixture safety slice extracted from #173. It changes one test file and does not alter production behavior.

Validation

• npx vitest run --config config/vitest.config.ts test/quick-start.test.ts: 15 passed
• npm run build: passed
• Commit formatting hook: passed
• git diff --check: passed