Adds web/GUARDRAILS.md capturing the recurring CI-guardrail and
test-harness lessons in one place: the check:size (250 KB-gz largest
chunk) and check:colors (design-token) guards, the jsdom/storybook
vitest split, MSW onUnhandledRequest:"error" overrides, RTL
accessible-name collisions, Storybook nested-router/portal handling,
and the components/ui code-style split. Wires a pointer from CLAUDE.md.
All claims verified against the live scripts, ci.yaml, and src/test/.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
