fix(vim): vim-lsp detection + drop hang-prone echo lines

`autoload/nuwiki/lsp.vim` probed vim-lsp via `exists('*lsp#register_server')`
but autoload functions aren't loaded until first call — that check
always returned false on the dev script's setup, so the plugin
printed "no supported LSP client found" even after vim-lsp had been
cloned + added to the runtimepath.

vim-lsp's `plugin/lsp.vim` sets `g:lsp_loaded = 1` only when its
hard requirements (`json_encode`, `timers`, `lambda`) are met, so
that's the reliable presence flag. Switched to `exists('g:lsp_loaded')`.

Also replaced `echo` with `echomsg` in the generated `start-vim.sh`
vimrc — the two startup status lines were pushing past `'cmdheight'`
and triggering "Press ENTER or type command to continue", which made
non-interactive invocations exit before the buffer was usable.
`echomsg` writes to `:messages` history without the prompt.

Verified with `vim --clean -u $DEV/vimrc index.wiki`:
- `g:lsp_loaded = 1`
- `filetype = vimwiki`
- `:VimwikiTOC` defined (buffer-local), `b:did_ftplugin = 1`
- No "press ENTER" hang.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

start-vim.sh

@@ -157,8 +157,9 @@ augroup NuwikiStart
   autocmd FileType vimwiki call nuwiki#lsp#start()
 augroup END
 
-echo '[nuwiki-dev] ready — wiki root: $WIKI_DIR'
-echo '[nuwiki-dev] :LspStatus for client diagnostics, :messages for log'
+" Status messages go to :messages so they don't trigger "Press ENTER".
+echomsg '[nuwiki-dev] ready — wiki root: $WIKI_DIR'
+echomsg '[nuwiki-dev] :LspStatus for diagnostics, :messages for log'
 EOF
 }