Lower log level of noisy kernel.execute and CodeRunner.attach_output_queue logs in agent

The agent emits too many INFO-level logs for every code execution, cluttering the log output. Lower these to DEBUG (or TRACE) level.

Affected log statements (agent/kernel.py):

- kernel.execute(k:..., run_id:..., mode:..., opts:...) at Kernel.execute()
- CodeRunner.attach_output_queue(k:..., run_id:..., is running event set:...) at CodeRunner.attach_output_queue()

These fire on every execution and are diagnostic in nature, so they belong at DEBUG level rather than INFO.

JIRA Issue: BA-6845

Open agent/kernel.py and inspect Kernel.execute() and CodeRunner.attach_output_queue(), focusing on the two INFO-level statements named in the issue. Change those execution-time diagnostic messages to DEBUG or TRACE, then verify that they no longer appear at INFO level during code execution.

Written by the indexing model from the issue text.