Add label trigger functionality to Claude Code Action (#177)

- introduced a new input parameter `label_trigger` in `action.yml` to allow triggering actions based on specific labels applied to issues.
- Enhanced the context preparation and event handling in the code to support the new labled event.

test/permissions.test.ts

@@ -62,6 +62,7 @@ describe("checkWritePermissions", () => {
     inputs: {
       triggerPhrase: "@claude",
       assigneeTrigger: "",
+      labelTrigger: "",
       allowedTools: [],
       disallowedTools: [],
       customInstructions: "",