fix(install): resolve plugin root at source time to fix NuwikiInstall E484

<sfile> inside a command! body is evaluated at execution time, not at
definition time. When :NuwikiInstall is run interactively there is no
sourcing context, so <sfile> expands to nothing and the path collapses
to ./scripts/download_bin.vim — producing E484.

Fix: capture the path in s:plugin_root immediately while the script is
being sourced, then reference that variable from the command body.

Also hoist :NuwikiInstall above the early `finish` for Neovim so the
command is available in both editors (Neovim routes to
require('nuwiki.install').install(), Vim sources the download script).

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

plugin/nuwiki.vim

@@ -12,6 +12,21 @@ if exists('g:loaded_nuwiki')
 endif
 let g:loaded_nuwiki = 1
 
+" Resolve the plugin root NOW, while this script is being sourced and
+" <sfile> is still valid. Commands that need this path must reference
+" s:plugin_root — re-expanding <sfile> inside a command body evaluates
+" at execution time when there is no sourcing context, producing an
+" empty path (hence the E484 "Cannot open ./scripts/…" error).
+let s:plugin_root = fnamemodify(resolve(expand('<sfile>:p')), ':h:h')
+
+" :NuwikiInstall — install the nuwiki-ls binary without needing a
+" plugin-manager build hook. Available for both Vim and Neovim.
+if has('nvim')
+  command! -nargs=0 NuwikiInstall lua require('nuwiki.install').install()
+else
+  command! -nargs=0 NuwikiInstall execute 'source' fnameescape(s:plugin_root . '/scripts/download_bin.vim')
+endif
+
 if has('nvim')
   " Neovim path is initialised by `require('nuwiki').setup()`. Nothing else
   " to do at vimscript boot — wait until the user runs setup.
@@ -24,10 +39,6 @@ augroup nuwiki_vim_lsp_start
   autocmd FileType vimwiki call nuwiki#lsp#start()
 augroup END
 
-" :NuwikiInstall — convenience wrapper around scripts/download_bin.vim, so
-" users don't have to invoke a plugin-manager build hook by hand.
-command! -nargs=0 NuwikiInstall execute 'source' fnamemodify(resolve(expand('<sfile>:p')), ':h:h') . '/scripts/download_bin.vim'
-
 " Global entry-point mappings — work from any buffer, not just .wiki files.
 " Files are opened directly from config (no LSP required); the server
 " auto-starts via the FileType autocmd once the vimwiki buffer loads.