fix: use debug level properly (#13)

* Add debug logging mode for plugin

* Add log level support for plugin logging

* Add tests and CI checks

* Expand behavioral test coverage

* Add coverage reporting and Codecov upload

* Add repo guidance and Given/When/Then test comments

* Fix Codecov workflow guard

* Make Given/When/Then comments descriptive

* Fix session-context test typecheck

* Remove config.debug docs and schema

* Remove config.debug runtime support

Author: lan17