docs: add CI troubleshooting guide for mobile checks (#513)

Closes #442

Contributors hitting a failing check (TypeScript, lint, tests, Expo
config, dependency install) had no single place documenting the local
command to reproduce it or how to fix it.

- Add docs/ci-troubleshooting.md: the four local commands that mirror
  CI (npm install --legacy-peer-deps, npm run typecheck, npm run lint,
  npm test), plus fixes for the failure patterns actually reproduced
  while writing this guide in this repo:
  - Peer dependency conflicts requiring --legacy-peer-deps
  - Multiple lock files (pnpm-lock.yaml/bun.lock alongside
    package-lock.json) causing Expo tooling to pick the wrong package
    manager, observed as `spawn bun ENOENT` when `expo lint` tried to
    auto-install eslint via bun
  - TypeScript syntax-error cascades (JSX parent element / unclosed
    brace errors)
  - Jest "Unexpected token 'export'" from an untransformed ESM
    dependency (lucide-react-native) missing from
    transformIgnorePatterns
  - Empty test files ("must contain at least one test")
  - Async test timeouts from unmocked dependencies
  - expo-doctor's network-dependent config-schema check failing in
    offline/sandboxed environments
  - A "clean install" recovery sequence
  - What reviewers/GrantFox evaluators expect from a green PR
- Add "lint": "expo lint" to package.json so the guide (and
  CONTRIBUTING.md) can reference npm run lint consistently with
  npm run typecheck / npm test, instead of a bare npx invocation.
- Link the guide from README.md, CONTRIBUTING.md's PR checklist, and
  mobile-onboarding-checklist.md's new "Common CI Failures" pointer.

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

Author: habnark