Add CI workflows to enforce the build gates on every PR

[OmarAlJarrah]

Problem

There's no .github/workflows, so apiCheck, koverVerify (80% floor), detekt, ktlint, explicit-API, and allWarningsAsErrors run only locally — nothing enforces them on PRs.

Proposed change

Add .github/workflows/ci.yml on pull_request + push: checkout, actions/setup-java (temurin 21 — foojay auto-provisions 8/11), gradle/actions/setup-gradle with caching, then ./gradlew build (which already runs every gate).

Acceptance

• CI runs ./gradlew build on PRs
• Gates fail the build on violation

Dependencies

• Blocks Add a test-only R8 shrink-survival module with consumer keep-rules #72 — R8 shrink-survival module (CI job)
• Blocks Add a runnable example module as an end-to-end usability smoke test #73 — example-module smoke test (CI job)
• Blocks Decide on release automation before the first non-alpha tag #75 — release-automation decision (after CI lands)

---

Priority: high · Effort: small