fix: normalize ignore patterns cross-platform and use config for diff index path

Address Copilot review feedback:
- Normalize user-provided ignore patterns to forward slashes in ScanIgnore
- Read index from cfg.Output.JSON in diff command instead of hardcoded path
- Handle backslash normalization on non-Windows OSes where filepath.ToSlash is a no-op
- Update tests to cover backslash ignore patterns

Co-Authored-By: KaranG08 <KaranG08@users.noreply.github.com>
Co-Authored-By: Copilot <copilot@github.com>

Author: 3 people

internal/cli/diff.go

@@ -17,24 +17,29 @@ func newDiffCmd() *cobra.Command {
 		Use:   "diff",
 		Short: "Show changes since last index generation",
 		RunE: func(cmd *cobra.Command, args []string) error {
-			// 1. Read stacklit.json
-			data, err := os.ReadFile("stacklit.json")
+			// 1. Load config and read the index file.
+			cfg := config.Load(".")
+			indexPath := cfg.Output.JSON
+			if indexPath == "" {
+				indexPath = "stacklit.json"
+			}
+
+			data, err := os.ReadFile(indexPath)
 			if err != nil {
-				return fmt.Errorf("could not read stacklit.json: %w (run 'stacklit generate' first)", err)
+				return fmt.Errorf("could not read %s: %w (run 'stacklit generate' first)", indexPath, err)
 			}
 
 			var index schema.Index
 			if err := json.Unmarshal(data, &index); err != nil {
-				return fmt.Errorf("could not parse stacklit.json: %w", err)
+				return fmt.Errorf("could not parse %s: %w", indexPath, err)
 			}
 
 			storedHash := index.MerkleHash
 			if storedHash == "" {
-				return fmt.Errorf("stacklit.json has no merkle_hash; run 'stacklit generate' to rebuild")
+				return fmt.Errorf("%s has no merkle_hash; run 'stacklit generate' to rebuild", indexPath)
 			}
 
 			// 2. Walk current source files, excluding Stacklit's own generated outputs.
-			cfg := config.Load(".")
 			files, err := walker.Walk(".", cfg.ScanIgnore())
 			if err != nil {
 				return fmt.Errorf("failed to walk source files: %w", err)

internal/config/config.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"os"
 	"path/filepath"
+	"strings"
 )
 
 // Config holds the settings loaded from a .stacklitrc.json file.
@@ -74,14 +75,26 @@ func Load(root string) *Config {
 }
 
 // ScanIgnore returns ignore patterns plus Stacklit output files so generated artifacts
-// never feed back into the next scan.
+// never feed back into the next scan. All patterns are normalized to forward slashes
+// so they match the walker's normalized paths on every OS.
 func (c *Config) ScanIgnore() []string {
-	ignore := append([]string{}, c.Ignore...)
+	ignore := make([]string, 0, len(c.Ignore)+3)
+	for _, pat := range c.Ignore {
+		ignore = append(ignore, toSlash(pat))
+	}
 	for _, out := range []string{c.Output.JSON, c.Output.Mermaid, c.Output.HTML} {
 		if out == "" {
 			continue
 		}
-		ignore = append(ignore, filepath.ToSlash(out))
+		ignore = append(ignore, toSlash(out))
 	}
 	return ignore
 }
+
+// toSlash normalizes both OS path separators and literal backslashes to forward
+// slashes. filepath.ToSlash only converts os.PathSeparator which is '/' on
+// Unix, leaving Windows-style backslashes untouched on non-Windows builds.
+func toSlash(p string) string {
+	p = filepath.ToSlash(p)
+	return strings.ReplaceAll(p, "\\", "/")
+}

internal/config/config_test.go

@@ -57,13 +57,13 @@ func TestLoadMalformed(t *testing.T) {
 
 func TestScanIgnoreIncludesOutputs(t *testing.T) {
 	cfg := DefaultConfig()
-	cfg.Ignore = []string{"custom/"}
+	cfg.Ignore = []string{"custom/", "vendor\\libs"}
 	cfg.Output.JSON = "out\\stacklit.json"
 	cfg.Output.Mermaid = "docs\\DEPENDENCIES.md"
 	cfg.Output.HTML = "stacklit.html"
 
 	got := cfg.ScanIgnore()
-	want := []string{"custom/", "out/stacklit.json", "docs/DEPENDENCIES.md", "stacklit.html"}
+	want := []string{"custom/", "vendor/libs", "out/stacklit.json", "docs/DEPENDENCIES.md", "stacklit.html"}
 	if len(got) != len(want) {
 		t.Fatalf("expected %d ignore patterns, got %d: %v", len(want), len(got), got)
 	}