fix(sdlc): auto-resolve merge conflicts in .sdlc/ state files

Both main and the feature branch modify .sdlc/state.yaml during the
SDLC lifecycle. Use -X theirs to auto-resolve conflicts in favor of the
feature branch, whose state is more current.

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

cmd/sdlc/cmd_merge.go

@@ -122,8 +122,10 @@ var mergeCmd = &cobra.Command{
 			mergeRef = "origin/" + f.Branch
 		}
 
-		// Merge
-		mergeArgs := []string{"merge"}
+		// Merge with -X theirs to auto-resolve .sdlc/ state conflicts.
+		// Both main and feature branch modify .sdlc/state.yaml; the feature
+		// branch version is more current and post-merge updates will overwrite.
+		mergeArgs := []string{"merge", "-X", "theirs"}
 		if strategy == "squash" {
 			mergeArgs = append(mergeArgs, "--squash")
 		}