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>

development/tests/test-coc-register-vim.vim

@@ -0,0 +1,42 @@
+" development/tests/test-coc-register-vim.vim — programmatic coc.nvim
+" registration. Relies on a stub autoload/coc.vim (created by the wrapper) that
+" records coc#config calls into g:_recorded. Drives nuwiki#lsp#start()'s coc
+" branch and asserts the exact languageserver config nuwiki injects (command,
+" filetypes, and the s:settings() payload with the global shorthand folded in).
+" Writes PASS/FAIL to $NUWIKI_COC_OUT.
+
+let s:out = []
+function! s:check(desc, cond) abort
+  call add(s:out, (a:cond ? 'PASS ' : 'FAIL ') . a:desc)
+endfunction
+
+" Pretend coc's node service is already up so registration runs synchronously.
+let g:coc_service_initialized = 1
+let g:_recorded = {}
+
+" Make sure the vim-lsp branch is NOT taken.
+if exists('g:lsp_loaded') | unlet g:lsp_loaded | endif
+
+" A readable, executable 'binary' so nuwiki#lsp#start() gets past its check.
+let g:nuwiki_binary_path = executable('true') ? exepath('true') : '/usr/bin/true'
+
+" Config: single-wiki shorthand + a global display setting that must fold into
+" the wiki the server sees (the global shorthand).
+let g:nuwiki_wiki_root = '/tmp/realwiki'
+let g:nuwiki_toc_header_level = 2
+
+call nuwiki#lsp#start()
+
+let s:r = get(g:, '_recorded', {})
+call s:check('coc#config was called', !empty(s:r))
+call s:check('section is languageserver.nuwiki', get(s:r, 'section', '') ==# 'languageserver.nuwiki')
+let s:v = get(s:r, 'value', {})
+call s:check('command is the binary', get(s:v, 'command', '') ==# g:nuwiki_binary_path)
+call s:check('filetypes = [vimwiki]', get(s:v, 'filetypes', []) == ['vimwiki'])
+let s:io = get(s:v, 'initializationOptions', {})
+call s:check('initOpts has wiki_root', get(s:io, 'wiki_root', '') ==# '/tmp/realwiki')
+call s:check('global toc_header_level folded into payload', get(s:io, 'toc_header_level', 0) == 2)
+call s:check('settings.nuwiki mirrors initOpts', get(get(s:v, 'settings', {}), 'nuwiki', {}) ==# s:io)
+
+call writefile(s:out, $NUWIKI_COC_OUT)
+qa!