feat(lsp): add list.removeCheckbox and link.catUrl commands

removeCheckbox strips the `[ ]` marker (plus its trailing space) from
the current item or every item in the list. catUrl returns the
`file://` URL of the current page's HTML export, mirroring vimwiki's
:VimwikiCatUrl.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/nuwiki-lsp/tests/commands_links.rs

@@ -28,7 +28,11 @@ fn parse(src: &str) -> nuwiki_core::ast::DocumentNode {
 #[test]
 fn commands_list_advertises_link_helpers() {
     let names: Vec<&str> = nuwiki_lsp::commands::COMMANDS.to_vec();
-    for name in ["nuwiki.link.pasteWikilink", "nuwiki.link.pasteUrl"] {
+    for name in [
+        "nuwiki.link.pasteWikilink",
+        "nuwiki.link.pasteUrl",
+        "nuwiki.link.catUrl",
+    ] {
         assert!(names.contains(&name), "missing: {name}");
     }
 }