fix(linkindex): stop trimming leading/trailing hyphens from GithubSlug

internal/linkindex/linkindex.go

@@ -136,14 +136,18 @@ var (
 	whitespaceRunRE = regexp.MustCompile(`\s+`)
 )
 
-// GithubSlug replicates GitHub's heading-anchor slugger: lowercase; strip all
-// but letters/digits/underscore/whitespace/hyphen; each whitespace char
-// becomes one hyphen; trim leading/trailing hyphens.
+// GithubSlug replicates GitHub's heading-anchor slugger (github-slugger, the
+// package GitHub itself extracted from its rendering pipeline): lowercase;
+// strip all but letters/digits/underscore/whitespace/hyphen; each whitespace
+// char becomes one hyphen. Nothing is trimmed -- a leading/trailing hyphen
+// survives whether it came from stripped punctuation ("`--json` output"
+// becomes "--json-output", not "json-output") or from literal leading/
+// trailing whitespace in the heading text, matching the reference
+// implementation, which calls no trim at all.
 func GithubSlug(heading string) string {
 	s := strings.ToLower(heading)
 	s = nonSlugRE.ReplaceAllString(s, "")
-	s = whitespaceRE.ReplaceAllString(s, "-")
-	return strings.Trim(s, "-")
+	return whitespaceRE.ReplaceAllString(s, "-")
 }
 
 // ConfluenceSlug replicates Confluence's scheme: preserve case and

internal/linkindex/slug_test.go

@@ -6,12 +6,13 @@ func TestGithubSlug(t *testing.T) {
 	cases := map[string]string{
 		"Hello World":        "hello-world",
 		"Hello, World!":      "hello-world",
-		"  Leading/trailing": "leadingtrailing",
-		" Hello ":            "hello",
-		"Café Menu":          "café-menu", // Unicode letters are preserved, not stripped
+		"  Leading/trailing": "--leadingtrailing", // leading whitespace is not trimmed
+		" Hello ":            "-hello-",           // nor is trailing
+		"Café Menu":          "café-menu",         // Unicode letters are preserved, not stripped
 		"under_score":        "under_score",
 		"":                   "",
-		"---":                "", // trims to nothing
+		"---":                "---",           // hyphens already in the heading are not trimmed either
+		"`--json` output":    "--json-output", // punctuation-derived hyphens survive too (#120)
 	}
 	for in, want := range cases {
 		if got := GithubSlug(in); got != want {