refactor: split command handlers, add focused tests, and add PR CI

[jaromil]

Summary

• split src/jaro responsibilities by moving command handlers into domain modules under src/zlibs/*
• added focused regression scripts for source smoke, addressbook/parse, filtering, and helper binaries
• documented maintenance boundaries and command/domain contracts
• added GitHub Actions workflow to run the full test suite on pull requests

Validation

• make
• extras/test/run-source.sh
• extras/test/test-addressbook-parse.sh
• extras/test/test-filtering.sh
• extras/test/test-helpers.sh

Notes

• .gestalt/ planning files are intentionally excluded from commits.