chore: import upstream snapshot with attribution

internal/mcp/diff_preview.go

@@ -0,0 +1,27 @@
+package mcp
+
+import (
+	"github.com/pmezard/go-difflib/difflib"
+)
+
+// unifiedDiff renders a standard unified diff (3 lines of context) between
+// oldContent and newContent, labelled with the given repo-relative path. It
+// powers the dry_run previews of the edit tools so an agent can see exactly
+// what a write would change — and review it — before committing the edit.
+// Returns "" when the inputs are identical or a diff cannot be produced.
+func unifiedDiff(path, oldContent, newContent string) string {
+	if oldContent == newContent {
+		return ""
+	}
+	out, err := difflib.GetUnifiedDiffString(difflib.UnifiedDiff{
+		A:        difflib.SplitLines(oldContent),
+		B:        difflib.SplitLines(newContent),
+		FromFile: "a/" + path,
+		ToFile:   "b/" + path,
+		Context:  3,
+	})
+	if err != nil {
+		return ""
+	}
+	return out
+}