label_trigger used a case-sensitive exact comparison, so a workflow
configured with label_trigger: "claude-task" did not fire when an issue
received a label named "Claude-Task" (the same label name with different
casing).
GitHub label names are unique without regard to case, so comparing without
case is unambiguous. It also matches the trigger_phrase check in the same
function, which is already case-insensitive.
Compare labelName and labelTrigger with toLowerCase(), and add a test
covering a mixed-case label.
Fixes#1571
