fix: remove race condition in cmd_delegate msg file deletion

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

skills/kugetsu/scripts/kugetsu-session.sh

@@ -228,7 +228,6 @@ cmd_delegate() {
     local msg_file="$LOGS_DIR/msg-$new_session.txt"
     printf '%s' "$message" > "$msg_file"
     nohup sh -c "GITEA_TOKEN='${GITEA_TOKEN:-}' opencode run '@$msg_file' --session '$new_session'" >> "$log_file" 2>&1 &
-    rm -f "$msg_file"
     echo "Delegated to new session (logged to $(basename "$log_file"))"
 }