test: add JaCoCo coverage reporting + CI test gate

[trinhvandat]

Problem

CLAUDE.md states a 80% test coverage target, but there is no way to measure or enforce it:

• No JaCoCo (or any coverage) plugin configured in pom.xml
• No .github/workflows/ directory — ./mvnw test is not run in CI at all, so nothing blocks a PR that drops coverage or breaks tests

Proposed changes

• Add jacoco-maven-plugin to pom.xml with a coverage report bound to verify
• Add a GitHub Actions workflow (.github/workflows/ci.yml) that runs ./mvnw test on PRs targeting develop/main
• Optionally enforce a minimum coverage threshold via JaCoCo's check goal once real test coverage exists (see #testing-strategy tracking issues)

Why this matters

Without CI enforcement, the 80% target in CLAUDE.md is aspirational only — there's currently ~0% coverage (see companion issues) and no guardrail to prevent regression once tests are added.