Set Up a CI Pipeline

Background: Native [iOS / Android / both] app. Repo on [GitHub / GitLab /
Bitbucket]. Tests exist: [unit / UI / none yet, per Section 8]. CI today:
[none / basic].

Use case: Set up continuous integration that builds and tests every pull
request so broken code never merges.

Implementation details:
- Recommend a CI for my setup (GitHub Actions, Xcode Cloud, Bitrise,
  GitLab CI) and why
- On every PR: build, run unit tests, run lint/format checks, report
  status back to the PR
- iOS: build with the right Xcode/scheme, run on a simulator, cache
  dependencies (SwiftPM/CocoaPods). Android: Gradle build + unit tests,
  cache Gradle, run on JVM where possible
- Keep it fast (caching, parallelization, run UI tests less often than
  unit)
- Fail the build on test failure or lint violations

Deliverable: A concrete CI config for my platform(s) and provider, the
PR-triggered jobs (build/test/lint), the caching strategy, and what gates
a merge.