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/new_test.go

@@ -85,7 +85,7 @@ func TestNewCollisions(t *testing.T) {
 }
 
 func TestNewRejectsBadTitles(t *testing.T) {
-	for _, title := range []string{"", "  ", "a\nb", "!!!"} {
+	for _, title := range []string{"", "  ", "a\nb", "!!!", "Drop C #", "#", "C ##"} {
 		if path, err := New(t.TempDir(), "adr", title, now); err == nil {
 			t.Errorf("title %q created %s", title, path)
 		}