ci: gate PRs with Playwright e2e + fix release-branch triggers

[DDecoene]

Two real CI holes, fixed before the 1.0.0 release:

1. CI never ran Playwright — only npm test (vitest) + build. That's how the More built-in functions: ROUND(), MOD(), MAX(), MIN(), TIME(), YEAR()/MONTH()/DAY() #4 built-ins shipped broken (unit-green, e2e-red, CI blind).
2. CI only triggered on main. Under GitFlow every PR targets release/**, so PRs feat: implement ? / ?? print command (#2) #21–test+fix: parity-line e2e coverage, and fix #4 built-ins unreachable via REPL #25 ran zero automated checks.

Changes

• .github/workflows/ci.yml: trigger on push/PR to main and release/**; add an e2e job (playwright install --with-deps chromium → playwright test), uploading the report artifact on failure.
• playwright.config.ts: add a webServer block so the suite auto-starts the dev server (required in CI; reuses an already-running server locally). Verified locally — npx playwright test with no manual server starts one and passes.
• CLAUDE.md DoD: both vitest and Playwright must pass; every user-facing feature ships an e2e case in the same PR; CI gates merge.

Targets release/v1.0.0 per GitFlow. This same CI config propagates to release/v1.1.0 when 1.0.0 reaches main.