feat(linter): add SortFile for sort-only README fixing

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/linter/fixer.go

@@ -145,3 +145,84 @@ func FixFile(path string) (int, error) {
 	w.WriteString("\n")
 	return fixCount, w.Flush()
 }
+
+// SortFile reads the README, sorts entries alphabetically within each section,
+// and writes the result back. Unlike FixFile, it does not modify descriptions
+// (no capitalization, period, or attribution changes).
+func SortFile(path string) (int, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return 0, err
+	}
+	defer f.Close()
+
+	var lines []string
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		lines = append(lines, scanner.Text())
+	}
+	if err := scanner.Err(); err != nil {
+		return 0, err
+	}
+
+	fixCount := 0
+
+	var headingLines []int
+	for i, line := range lines {
+		if sectionHeadingRe.MatchString(line) {
+			headingLines = append(headingLines, i)
+		}
+	}
+
+	for i, headingIdx := range headingLines {
+		start := headingIdx + 1
+		end := len(lines)
+		if i+1 < len(headingLines) {
+			end = headingLines[i+1]
+		}
+
+		var entryPositions []int
+		var entries []parser.Entry
+		for lineIdx := start; lineIdx < end; lineIdx++ {
+			entry, err := parser.ParseEntry(lines[lineIdx], lineIdx+1)
+			if err != nil {
+				continue
+			}
+			entryPositions = append(entryPositions, lineIdx)
+			entries = append(entries, entry)
+		}
+		if len(entries) == 0 {
+			continue
+		}
+
+		sorted := SortEntries(entries)
+		for j, e := range sorted {
+			lineIdx := entryPositions[j]
+			// Use the original Raw line from the sorted entry to preserve formatting
+			if lines[lineIdx] != e.Raw {
+				fixCount++
+				lines[lineIdx] = e.Raw
+			}
+		}
+	}
+
+	if fixCount == 0 {
+		return 0, nil
+	}
+
+	out, err := os.Create(path)
+	if err != nil {
+		return 0, err
+	}
+	defer out.Close()
+
+	w := bufio.NewWriter(out)
+	for i, line := range lines {
+		w.WriteString(line)
+		if i < len(lines)-1 {
+			w.WriteString("\n")
+		}
+	}
+	w.WriteString("\n")
+	return fixCount, w.Flush()
+}

internal/linter/fixer_test.go

@@ -139,6 +139,94 @@ Some text here.
 	}
 }
 
+func TestSortFile(t *testing.T) {
+	content := `# Awesome Docker
+
+## Tools
+
+- [Zebra](https://example.com/zebra) - a tool by [@author](https://github.com/author)
+- [Alpha](https://example.com/alpha) - another tool
+
+## Other
+
+Some text here.
+`
+	tmp, err := os.CreateTemp("", "readme-sort-*.md")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.Remove(tmp.Name())
+
+	if _, err := tmp.WriteString(content); err != nil {
+		t.Fatal(err)
+	}
+	tmp.Close()
+
+	count, err := SortFile(tmp.Name())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if count == 0 {
+		t.Fatal("expected fixes, got 0")
+	}
+
+	data, err := os.ReadFile(tmp.Name())
+	if err != nil {
+		t.Fatal(err)
+	}
+	result := string(data)
+
+	// Check sorting: Alpha should come before Zebra
+	alphaIdx := strings.Index(result, "[Alpha]")
+	zebraIdx := strings.Index(result, "[Zebra]")
+	if alphaIdx > zebraIdx {
+		t.Error("expected Alpha before Zebra after sort")
+	}
+
+	// SortFile must NOT capitalize descriptions
+	if strings.Contains(result, "- A tool") {
+		t.Errorf("SortFile should not capitalize descriptions, got:\n%s", result)
+	}
+
+	// SortFile must NOT remove attribution
+	if !strings.Contains(result, "@author") {
+		t.Errorf("SortFile should preserve attribution, got:\n%s", result)
+	}
+
+	// SortFile must NOT add periods
+	if strings.Contains(result, "another tool.") {
+		t.Errorf("SortFile should not add periods, got:\n%s", result)
+	}
+}
+
+func TestSortFileIdempotent(t *testing.T) {
+	content := `# Awesome Docker
+
+## Tools
+
+- [Alpha](https://example.com/alpha) - A tool.
+- [Bravo](https://example.com/bravo) - B tool.
+`
+	tmp, err := os.CreateTemp("", "readme-sort-idem-*.md")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.Remove(tmp.Name())
+
+	if _, err := tmp.WriteString(content); err != nil {
+		t.Fatal(err)
+	}
+	tmp.Close()
+
+	count, err := SortFile(tmp.Name())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if count != 0 {
+		t.Fatalf("expected no changes on already-sorted file, got %d", count)
+	}
+}
+
 func TestFixFileSortsAcrossBlankLinesAndIsIdempotent(t *testing.T) {
 	content := `# Awesome Docker