Adds JaCoCo 0.8.12 with prepare-agent, report, and check executions.
Baseline measured at 46.8% branch coverage. Gate set at 42% (baseline
minus 5%) to prevent regression while giving room to close the gap.
Excluded from measurement: DTOs, config classes, model entities,
ErrorCode enum — these contain no testable branch logic.
Target is 80%; gap documented in issue #120.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>