fix: avoid --delete-branch to prevent worktree branch deletion error

gh pr merge --delete-branch fails when the current worktree is on the
branch being merged, because git refuses to delete a checked-out branch.
Instead, merge without --delete-branch and delete only the remote branch
afterward. Local branch cleanup is left to the worktree manager.

src/main/github/client.ts

@@ -249,11 +249,34 @@ export async function mergePR(
 ): Promise<{ ok: true } | { ok: false; error: string }> {
   await acquire()
   try {
-    await execFileAsync('gh', ['pr', 'merge', String(prNumber), `--${method}`, '--delete-branch'], {
+    await execFileAsync('gh', ['pr', 'merge', String(prNumber), `--${method}`], {
       cwd: repoPath,
       encoding: 'utf-8',
       env: { ...process.env, GH_PROMPT_DISABLED: '1' }
     })
+    // Delete remote branch separately — local branch deletion fails when
+    // the user's worktree is checked out on that branch, so we only clean
+    // up the remote side. Local cleanup is left to the caller / worktree manager.
+    const headBranch = (
+      await execFileAsync(
+        'gh',
+        ['pr', 'view', String(prNumber), '--json', 'headRefName', '-q', '.headRefName'],
+        {
+          cwd: repoPath,
+          encoding: 'utf-8'
+        }
+      )
+    ).stdout.trim()
+    if (headBranch) {
+      try {
+        await execFileAsync('git', ['push', 'origin', '--delete', headBranch], {
+          cwd: repoPath,
+          encoding: 'utf-8'
+        })
+      } catch {
+        // Remote branch may already be deleted by GitHub — ignore
+      }
+    }
     return { ok: true }
   } catch (err) {
     const message =