cmd_delegate deletes the message file immediately after spawning the
background nohup process, causing a race condition where opencode run
may not have read the file yet.
Error: File not found: msg-ses_xxx.txt
Fix by removing rm -f "$msg_file", consistent with cmd_start and
cmd_continue which write to $worktree_path/.kugetsu-msg.txt and never
delete it. Orphaned msg files in $LOGS_DIR/ are harmless.
Fixes#210
