chore(mobile-expo): add CI lint, typecheck, and build for Expo app

[up-tandem]

Background

The Expo app (@forge/expo) currently has no CI pipeline. Other mobile targets (iOS, Android) already have dedicated CI jobs in .github/workflows/ci.yml, but Expo is missing.

Expected Outcome

CI runs lint, typecheck, and build for the Expo app on affected PRs, matching the pattern used for iOS and Android.

Acceptance Criteria

• affected job detects @forge/expo changes via Turborepo
• New lint-expo CI job runs lint (--max-warnings=0), typecheck, and build
• Expo package is excluded from the monorepo-wide lint job (like other mobile packages)
• CI passes on PRs that touch only Expo code

Possible Solution

• Add mobile_expo output to the affected job
• Add @forge/expo to the affected package filter
• Exclude @forge/expo from the monorepo-wide lint step
• Add a new lint-expo job gated on needs.affected.outputs.mobile_expo == 'true'

References

• .github/workflows/ci.yml
• mobile/expo/ (Expo app workspace)