fix(lsp): resolve wikilinks that include the .wiki file extension

`[[page.wiki]]` should resolve identically to `[[page]]`. The index
keyed by stem, so links with the explicit extension fell through to
the broken-link diagnostic and to-page navigation failed. Strip the
configured extension before every page lookup (resolve, definition,
backlinks, classify_link, classify_outgoing) and stop appending the
extension in synthesise_page_uri when it's already present.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/nuwiki-lsp/src/wiki.rs

@@ -26,7 +26,8 @@ pub struct Wiki {
 impl Wiki {
     pub fn new(id: WikiId, config: WikiConfig) -> Self {
         let index = WorkspaceIndex::new(Some(config.root.clone()))
-            .with_diary_rel_path(Some(config.diary_rel_path.clone()));
+            .with_diary_rel_path(Some(config.diary_rel_path.clone()))
+            .with_file_extension(Some(config.file_extension.clone()));
         let index = Arc::new(RwLock::new(index));
         Self { id, config, index }
     }