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.

README.md

@@ -49,7 +49,7 @@ on:
   pull_request_review_comment:
     types: [created]
   issues:
-    types: [opened, assigned]
+    types: [opened, assigned, labeled]
   pull_request_review:
     types: [submitted]
 
@@ -65,6 +65,8 @@ jobs:
           # trigger_phrase: "/claude"
           # Optional: add assignee trigger for issues
           # assignee_trigger: "claude"
+          # Optional: add label trigger for issues
+          # label_trigger: "claude"
           # Optional: add custom environment variables (YAML format)
           # claude_env: |
           #   NODE_ENV: test
@@ -92,6 +94,7 @@ jobs:
 | `custom_instructions` | Additional custom instructions to include in the prompt for Claude                                                   | No       | ""        |
 | `mcp_config`          | Additional MCP configuration (JSON string) that merges with the built-in GitHub MCP servers                          | No       | ""        |
 | `assignee_trigger`    | The assignee username that triggers the action (e.g. @claude). Only used for issue assignment                        | No       | -         |
+| `label_trigger`       | The label name that triggers the action when applied to an issue (e.g. "claude")                                     | No       | -         |
 | `trigger_phrase`      | The trigger phrase to look for in comments, issue/PR bodies, and issue titles                                        | No       | `@claude` |
 | `claude_env`          | Custom environment variables to pass to Claude Code execution (YAML format)                                          | No       | ""        |