feat: Expose GitHub token as action output for external use

This allows workflows to use the Claude App token obtained by the action
for posting comments as claude[bot] instead of github-actions[bot].

Changes:
- Add github_token output to action.yml
- Export token from prepare.ts after authentication
- Allows workflows to use the same token Claude uses internally

src/entrypoints/prepare.ts

@@ -61,6 +61,9 @@ async function run() {
     });
 
     // MCP config is handled by individual modes (tag/agent) and included in their claude_args output
+    
+    // Expose the GitHub token (Claude App token) as an output
+    core.setOutput("github_token", githubToken);
 
     // Step 6: Get system prompt from mode if available
     if (mode.getSystemPrompt) {