Use Amp's exit summary for task commit message

When the 'amp' subprocess completes, it provides a summary of the work done. Currently, Omni.Agent.Worker uses a generic commit message ('feat: implement <tid>'). Modify Omni.Agent.Worker to capture the output/summary from 'amp' and use it as the git commit message. You may need to adjust how 'amp' is invoked or read a specific output file if 'amp' writes the summary to disk.