feat(cmd): add unified diff output option for formatting

* Implemented `-d`/`--diff` flag to print unified diffs.
* Added `unifiedDiff` function for generating diffs.
* Config discovery now merges fields from `.pgtidy.yaml`.

cmd/pgtidy/fmt.go

@@ -12,12 +12,14 @@ import (
 
 // cmdFmt implements `pgtidy fmt`. It follows the gofmt model: with no flags it
 // prints the formatted result to stdout; -w rewrites in place; -l lists files
-// that differ; --check exits non-zero if any input is unformatted.
+// that differ; --check exits non-zero if any input is unformatted; -d prints a
+// unified diff for each file that would change.
 func cmdFmt(args []string, stdin io.Reader, stdout, stderr io.Writer) int {
 	var (
 		write bool
 		list  bool
 		check bool
+		diff  bool
 		files []string
 	)
 	for _, a := range args {
@@ -28,6 +30,8 @@ func cmdFmt(args []string, stdin io.Reader, stdout, stderr io.Writer) int {
 			list = true
 		case "--check":
 			check = true
+		case "-d", "--diff":
+			diff = true
 		case "-h", "--help":
 			usage(stdout)
 			return 0
@@ -40,8 +44,16 @@ func cmdFmt(args []string, stdin io.Reader, stdout, stderr io.Writer) int {
 		}
 	}
 
-	// TODO: discover and parse .pgtidy.yaml; for now use the house-style default.
-	st := config.Default()
+	// Discover config: walk up from the working directory.
+	wd, err := os.Getwd()
+	if err != nil {
+		wd = "."
+	}
+	st, err := config.Load(wd)
+	if err != nil {
+		fmt.Fprintf(stderr, "%v\n", err)
+		return 2
+	}
 
 	// stdin → stdout when no files are given.
 	if len(files) == 0 {
@@ -51,13 +63,16 @@ func cmdFmt(args []string, stdin io.Reader, stdout, stderr io.Writer) int {
 			return 2
 		}
 		out := format.File(parser.Parse(string(src)), st)
-		if check {
+		switch {
+		case check:
 			if out != string(src) {
 				return 1
 			}
-			return 0
+		case diff:
+			io.WriteString(stdout, unifiedDiff(string(src), out, "stdin"))
+		default:
+			io.WriteString(stdout, out)
 		}
-		io.WriteString(stdout, out)
 		return 0
 	}
 
@@ -87,6 +102,10 @@ func cmdFmt(args []string, stdin io.Reader, stdout, stderr io.Writer) int {
 			if changed {
 				fmt.Fprintln(stdout, path)
 			}
+		case diff:
+			if changed {
+				io.WriteString(stdout, unifiedDiff(string(src), out, path))
+			}
 		case check:
 			// handled after loop via anyDiff
 		default: