fix: support custom anchor IDs in headings ({#my-anchor} syntax)

Add parser.WithAttribute() to the goldmark parser options so that
extended Markdown heading attributes like {#custom-id} are parsed
correctly instead of appearing as literal text in the heading.

Before: # My Heading {#custom-id} → <h1>My Heading {#custom-id}</h1>
After:  # My Heading {#custom-id} → <h1 id="custom-id">My Heading</h1>

Fixes #519

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: mrueg

markdown/markdown.go

@@ -109,6 +109,7 @@ func compileMarkdownWithExtension(markdown []byte, ext goldmark.Extender, logMes
 		),
 		goldmark.WithParserOptions(
 			parser.WithAutoHeadingID(),
+			parser.WithAttribute(),
 		),
 		goldmark.WithRendererOptions(
 			html.WithUnsafe(),