pass logdir path to support downstream logging

[Priyank-Shethia3]

Description

Add optional log_dir parameter to agent runner interface to enable external agents to write output files to the correct location.

Closes #

Changes

• Added log_dir: str | None parameter to AgentRunner Protocol in agents/__init__.py
• Updated execute_coop() and _spawn_agent() in runner/coop.py to compute and pass log directory path based on run_name and features
• Updated execute_solo() and _spawn_solo_agent() in runner/solo.py to compute and pass log directory path based on run_name and features
• All changes are backward compatible - existing agents ignore the parameter, external agents can opt-in

Checklist

• Code follows project style
• Tests pass locally (34/34 unit tests passing)
• Documentation updated (docstrings added for new parameter)

[akhatua2]

LGTM