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.sh

@@ -0,0 +1,71 @@
+#!/usr/bin/env bash
+#
+# development/tests/test-coc-register-vim.sh — programmatic coc.nvim
+# registration for the Vim client.
+#
+# nuwiki#lsp#start() registers the language server with coc via coc#config so
+# users don't hand-maintain coc-settings.json. This harness stubs coc#config
+# (a real autoload/coc.vim on the runtimepath, since Vim's exists('*coc#config')
+# goes through the autoload mechanism) to record what nuwiki injects, then runs
+# test-coc-register-vim.vim and asserts the payload.
+#
+# Exit code: 0 when every check passes, 1 on any FAIL or missing output.
+# Skips (exit 0) when vim is not installed.
+
+set -euo pipefail
+
+REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)"
+
+log() { printf '\033[1;34m[coc-register]\033[0m %s\n' "$*"; }
+
+if ! command -v vim >/dev/null 2>&1; then
+  log 'vim not installed — skipping'
+  exit 0
+fi
+
+TMP="$(mktemp -d -t nuwiki-coc-test-XXXXXX)"
+trap 'rm -rf "$TMP"' EXIT
+
+# Stub coc#config as a real autoload function so exists('*coc#config') resolves
+# it. It records the last call into g:_recorded for the test to assert.
+mkdir -p "$TMP/stub/autoload"
+cat > "$TMP/stub/autoload/coc.vim" <<'EOF'
+function! coc#config(section, value) abort
+  let g:_recorded = {'section': a:section, 'value': deepcopy(a:value)}
+endfunction
+EOF
+
+OUT="$TMP/coc.out"
+VIMRC="$TMP/vimrc"
+cat > "$VIMRC" <<EOF
+set nocompatible
+" Stub dir first so its autoload/coc.vim is the one that resolves.
+let &runtimepath = '$TMP/stub' . ',' . '$REPO_ROOT' . ',' . &runtimepath
+" coc.nvim defines :CocConfig at startup; nuwiki uses that as the reliable
+" 'coc is present' signal. Stub it so the coc branch is taken.
+command! -nargs=0 CocConfig echo ''
+EOF
+
+log 'running Vim coc-registration harness…'
+NUWIKI_COC_OUT="$OUT" \
+  timeout 30 vim -e -s -u "$VIMRC" \
+    -c "source $REPO_ROOT/development/tests/test-coc-register-vim.vim" \
+    </dev/null >"$TMP/vim.log" 2>&1 || true
+
+if [[ ! -f "$OUT" ]]; then
+  echo 'coc-registration harness produced no output' >&2
+  echo '--- vim log ---' >&2
+  cat "$TMP/vim.log" >&2 || true
+  exit 1
+fi
+
+cat "$OUT"
+PASSED="$(grep -c '^PASS ' "$OUT" || true)"
+FAILED="$(grep -c '^FAIL ' "$OUT" || true)"
+echo "SUMMARY: ${PASSED} passed, ${FAILED} failed"
+
+if [[ "$FAILED" -ne 0 ]]; then
+  exit 1
+fi
+log 'coc registration OK ✓'
+exit 0

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!