Remove heartbeat from exec-based Claude execution

[mcintyre94]

Summary

• Removes the 20s stderr heartbeat that was added to keep sprites alive while Claude waits for API responses
• Exec sessions stay alive as long as we're connected and for up to an hour after disconnect, so it's no longer needed
• The heartbeat was specifically a workaround for the service-based approach where output inactivity could cause the sprite to pause

Test plan

• Send a chat message and verify Claude responds normally
• Let Claude do a slow tool call (e.g. a long bash command) and confirm the session doesn't drop
• Verify the exec session stays alive across a longer idle period

🤖 Generated with Claude Code