Enable automated package checks and regression tests on master #25

Context

After #22 and #24, the team requested automated test execution in AlertTools. Current master does not contain R-CMD-check CI. Release candidate PR #20 already introduces package checks on Linux/macOS/Windows, ephemeral PostgreSQL integration, coverage, lint and consumer migration validation.

This issue tracks delivery through #20 rather than a duplicate CI implementation. See docs/refactoring/master-integration-2026-10-01.md in that PR for integration evidence.

Acceptance criteria

• CI runs on pull requests and pushes to master.
• Dependencies are installed in clean runners.
• The complete suite includes firstday and calendar regression tests.
• The PostgreSQL integration job installs devtools explicitly.
• Failures and logs are visible in GitHub Actions.
• A successful remote run is recorded before delivery to master.

Local evidence on aaf7ab2: 297 passing expectations, 0 failures, 0 warnings, 1 PostgreSQL opt-in skip. R CMD build passed; R CMD check: 0 errors, 0 warnings, 1 NOTE (lintr unavailable locally). Remote results are pending.