Adds local_args field to HerdrCommand::SpawnAgent, enabling the
Local provider to pass argv to the agent binary without a wrapper
script. Backward-compatible — local_args defaults to None.
Real Pi spawn on FreeBSD is now:
spawn-agent provider=local model=pi local_args=['--mode','json','--no-tools','-p','task']
Previously required a wrapper script because only an executable
path was accepted. This closes the OSA wrapper caveat from PR #9.
Build: pass | Tests: workspace green
