fix(sdlc): use remote ref for merge when local branch doesn't exist

After resetToMain in the executor, only remote refs exist for feature
branches. The merge command now checks if the local branch exists and
falls back to origin/<branch> when it doesn't.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cmd/sdlc/cmd_merge.go

@@ -113,12 +113,21 @@ var mergeCmd = &cobra.Command{
 			_ = resetCmd.Run()
 		}
 
+		// Determine merge ref: prefer local branch, fall back to origin/<branch>
+		// when running in worker pods where resetToMain leaves only remote refs.
+		mergeRef := f.Branch
+		checkBranch := exec.Command("git", "rev-parse", "--verify", f.Branch)
+		checkBranch.Dir = root
+		if err := checkBranch.Run(); err != nil {
+			mergeRef = "origin/" + f.Branch
+		}
+
 		// Merge
 		mergeArgs := []string{"merge"}
 		if strategy == "squash" {
 			mergeArgs = append(mergeArgs, "--squash")
 		}
-		mergeArgs = append(mergeArgs, f.Branch)
+		mergeArgs = append(mergeArgs, mergeRef)
 
 		gitMerge := exec.Command("git", mergeArgs...)
 		gitMerge.Dir = root