Fix check_update_lock() to provide user guidance for potentially stale lock files (#133)

* Initial plan

* Add manual fix instructions for stale lock files in check_update_lock()

Co-authored-by: paolomainardi <8747+paolomainardi@users.noreply.github.com>

* Update lock file message to be less definitive about staleness per review feedback

Co-authored-by: paolomainardi <8747+paolomainardi@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: paolomainardi <8747+paolomainardi@users.noreply.github.com>

Author: Copilot

bin/sparkdock.macos

@@ -64,6 +64,11 @@ check_for_updates() {
         elif ps -p "$pid" > /dev/null; then
             print "Another update is in progress (PID: $pid)"
             exit 1
+        else
+            print_warning "Lock file exists but the process (PID: $pid) is no longer running."
+            print_warning "If you believe this is a stale lock file, you can remove it manually by running:"
+            print_warning "  rm -f $LOCK_FILE"
+            exit 1
         fi
     fi
     echo $$ > "$LOCK_FILE"