feat(FN-1236): migrate GitHub PR workflows to gh authentication

- Require gh-authenticated paths for task pr-create and dashboard PR routes
- Switch dashboard command and extension GitHub tools to use the gh auth client instead of token-based auth
- Update CLI and dashboard tests to cover gh-only createPr and import behavior
- Refresh README guidance and add a changeset removing the GitHub token requirement for PR flows

packages/cli/src/commands/dashboard.ts

@@ -395,7 +395,7 @@ export async function runDashboard(port: number, opts: { paused?: boolean; dev?:
   // pause is deduplicated across concurrent agents.
   //
   const usageLimitPauser = new UsageLimitPauser(store);
-  const githubClient = new GitHubClient(process.env.GITHUB_TOKEN);
+  const githubClient = new GitHubClient();
 
   // AI-powered merge handler (used by the web UI for manual merges).
   // Wrapped with the shared semaphore so merges count toward the global