feat(config): P3 config batch — group 2 (create_link, dir_link, auto_header)

- create_link (default true): server resolve_target_uri now only synthesises a
  missing-page URI when create_link is set; when off, following a link to a
  non-existent page returns no location (no-op).
- dir_link: a [[dir/]] link now resolves to <dir>/<dir_link><ext> when dir_link
  is set, else opens the directory itself. Uses the already-parsed
  LinkTarget.is_directory.
- auto_header (default off): new wiki pages get a `= Stem =` header derived from
  the filename, skipping the wiki index + diary index. Client-side BufNewFile
  autocmd in both clients (init.lua + plugin/nuwiki.vim), with auto_header
  handlers in both command layers (trailing-slash-robust root matching).

Tests: cmd.auto_header_inserts + cmd.auto_header_skips_diary_index (nvim) /
cmd.auto_header_skips_index (vim). Full rust suite + both harnesses green
(Neovim 307, Vim 301/18/21).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/nuwiki-lsp/src/lib.rs

@@ -269,7 +269,29 @@ impl Backend {
                         }
                     }
                     let path = target.path.as_deref()?;
-                    return synthesise_page_uri(&source_wiki.config, path);
+                    // vimwiki `dir_link`: a `[[dir/]]` link opens `<dir>/<dir_link>`
+                    // (e.g. `index` → `dir/index.wiki`); empty → the directory itself.
+                    if target.is_directory {
+                        let dl = source_wiki.config.dir_link.trim();
+                        if dl.is_empty() {
+                            let mut p = source_wiki.config.root.clone();
+                            for seg in path.split('/').filter(|s| !s.is_empty()) {
+                                p.push(seg);
+                            }
+                            return Url::from_file_path(p).ok();
+                        }
+                        let joined = format!("{}/{dl}", path.trim_end_matches('/'));
+                        return synthesise_page_uri(&source_wiki.config, &joined);
+                    }
+                    // vimwiki `create_link`: when off, only follow to a page that
+                    // already exists on disk — never synthesise a new one.
+                    let uri = synthesise_page_uri(&source_wiki.config, path)?;
+                    if !source_wiki.config.create_link
+                        && !uri.to_file_path().map(|p| p.exists()).unwrap_or(false)
+                    {
+                        return None;
+                    }
+                    return Some(uri);
                 }
                 // No wiki registered (no `initializationOptions`, no
                 // workspace folder). Fall back to the source buffer's