Handle malformed env files cleanly

[Dreamstore2046]

Summary

• replace the unified server's dotenv/config side effect import with a manual .env loader
• validate .env syntax before applying values, including malformed assignments and unbalanced quotes
• print Failed to parse .env: <message> and exit with status 1 when parsing fails
• document supported .env syntax in docs/setup/env-file.md

Closes #235

Validation

• npm test -- shared/__tests__/env-file.test.ts
• npx tsc --noEmit --target ES2022 --module ESNext --moduleResolution bundler --allowImportingTsExtensions --esModuleInterop --allowSyntheticDefaultImports --strict --skipLibCheck --types node,vitest shared/env-file.ts shared/load-env.ts shared/__tests__/env-file.test.ts
• git diff --cached --check

Existing build state

• npm run build still fails on pre-existing repository type errors unrelated to this PR, including missing @playwright/test, existing agent/test type errors, and existing service/factory type mismatches.

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