fix: reuse SSH agent across repeated action steps

[whyujjwal]

Summary

• reuse an existing SSH agent across repeated action steps instead of unconditionally starting a new one on the same socket
• remove stale sockets before starting a fresh agent and export SSH_AGENT_PID when a new agent is created
• add a regression test that reproduces two sequential action runs with the same SSH_AUTH_SOCK

Testing

• npm test
• npm run typecheck

Fixes #31