feat(coc): auto-register the language server with coc.nvim

coc users had to hand-maintain a coc-settings.json `languageserver.nuwiki`
entry, which didn't track g:nuwiki_wikis / g:vimwiki_list / the global
shorthand and meant editing JSON per wiki. nuwiki now registers itself
programmatically: on the first .wiki buffer it calls
coc#config('languageserver.nuwiki', {…}) with the s:settings() payload
(same config the vim-lsp path sends). coc reacts to the languageserver
config change (onDidChangeConfiguration → registerClientsByConfig) and
starts the server for the open buffer.

Details:
- Reliable coc detection via :CocConfig (exists('*coc#config') can't be
  trusted — it doesn't trigger autoload in Vim 9.2). The coc#config call
  is wrapped in try/catch and autoloads coc.vim itself; falls back to the
  printed snippet only if coc genuinely isn't there.
- Deferred to User CocNvimInit when coc's node service isn't up yet.
- Opt out with g:nuwiki_no_coc_register (then we just print the snippet).

Dogfooded in start-vim-coc.sh: dropped the manual coc-settings.json
languageserver block; the harness now relies on auto-registration from
g:nuwiki_* (real-user flow). New harness test-coc-register-vim (7 checks,
stubbed coc#config) asserts the injected payload incl. the folded global
shorthand; wired into CI. README coc section rewritten (zero-config).
Rust 573 passed, clippy clean, all harnesses green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

autoload/nuwiki/lsp.vim

@@ -136,22 +136,7 @@ function! nuwiki#lsp#start() abort
   endif
 
   if exists(':CocConfig') == 2 || exists('*coc#config')
-    " coc.nvim — coc reads its server list from coc-settings.json.
-    " We can't inject it dynamically without owning the file, so just point
-    " the user at the snippet. `initializationOptions` carries wiki_root etc.
-    " to the server exactly like the vim-lsp registration above — without it
-    " the server resolves links against its default root, so existing pages
-    " look broken and follow-to-create lands in the wrong directory.
-    echohl WarningMsg
-    echom 'nuwiki: coc.nvim detected. Add this to your coc-settings.json:'
-    echom '  "languageserver": {'
-    echom '    "nuwiki": {'
-    echom '      "command": "' . l:bin . '",'
-    echom '      "filetypes": ["vimwiki"],'
-    echom '      "initializationOptions": ' . json_encode(s:settings())
-    echom '    }'
-    echom '  }'
-    echohl None
+    call s:register_with_coc(l:bin)
     return
   endif
 
@@ -162,6 +147,66 @@ function! nuwiki#lsp#start() abort
   echohl None
 endfunction
 
+" ===== coc.nvim integration =====
+"
+" Register the nuwiki language server with coc *programmatically* via
+" coc#config, so users don't have to hand-maintain a coc-settings.json entry
+" per wiki. The config (wiki roots, per-wiki keys, the vimwiki/nuwiki global
+" shorthand) comes from s:settings() — the same payload the vim-lsp path sends.
+" coc reacts to the `languageserver` config change, registers the server, and
+" starts it for the open vimwiki buffer.
+"
+" Opt out with `let g:nuwiki_no_coc_register = 1` to manage the entry in
+" coc-settings.json yourself (we then just print the snippet).
+function! s:register_with_coc(bin) abort
+  if get(g:, 'nuwiki_no_coc_register', 0)
+    call s:print_coc_snippet(a:bin)
+    return
+  endif
+  let s:coc_bin = a:bin
+  if get(g:, 'coc_service_initialized', 0)
+    call s:coc_register()
+  else
+    " coc hasn't finished starting its node service yet — defer until it has,
+    " otherwise the updateConfig notification is sent to a service that isn't
+    " listening.
+    augroup nuwiki_coc_register
+      autocmd!
+      autocmd User CocNvimInit ++once call s:coc_register()
+    augroup END
+  endif
+endfunction
+
+function! s:coc_register() abort
+  let l:cfg = s:settings()
+  " Call coc#config directly — `exists('*coc#config')` can't be trusted because
+  " it doesn't trigger autoloading, so we let the call autoload coc.vim and
+  " fall back to the manual snippet only if coc genuinely isn't there.
+  try
+    call coc#config('languageserver.nuwiki', {
+          \ 'command':                s:coc_bin,
+          \ 'filetypes':              ['vimwiki'],
+          \ 'initializationOptions':  l:cfg,
+          \ 'settings':               { 'nuwiki': l:cfg },
+          \ })
+  catch /^Vim\%((\a\+)\)\=:E11[78]/
+    call s:print_coc_snippet(s:coc_bin)
+  endtry
+endfunction
+
+function! s:print_coc_snippet(bin) abort
+  echohl WarningMsg
+  echom 'nuwiki: coc.nvim detected. Add this to your coc-settings.json:'
+  echom '  "languageserver": {'
+  echom '    "nuwiki": {'
+  echom '      "command": "' . a:bin . '",'
+  echom '      "filetypes": ["vimwiki"],'
+  echom '      "initializationOptions": ' . json_encode(s:settings())
+  echom '    }'
+  echom '  }'
+  echohl None
+endfunction
+
 function! s:bin_path() abort
   if exists('g:nuwiki_binary_path') && filereadable(g:nuwiki_binary_path)
     return g:nuwiki_binary_path