fix: address docs checker review findings

Check every link reference definition, decode destinations per CommonMark,
report correct lines for empty headings, autolinks and nested definitions,
reject symlinked record directories and wrong-case path components, and
refuse titles that cannot pass. Only top-level headings count (spec section 6).

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/docs/check.go

@@ -57,10 +57,14 @@ func Check(root string) (Result, error) {
 	c := &checker{root: root, realRoot: real}
 	var recs []*record
 	for _, d := range dirs {
-		entries, err := os.ReadDir(filepath.Join(root, d.dir))
-		if os.IsNotExist(err) {
+		if st, err := os.Lstat(filepath.Join(root, d.dir)); os.IsNotExist(err) {
 			continue
-		} else if err != nil {
+		} else if err == nil && !st.IsDir() {
+			c.add(d.dir, 1, "must be a real directory (symlinks are not allowed)")
+			continue
+		}
+		entries, err := os.ReadDir(filepath.Join(root, d.dir))
+		if err != nil {
 			return Result{}, err
 		}
 		for _, e := range entries {