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/start-vim-coc.sh

@@ -72,46 +72,14 @@ ensure_coc() {
 
 write_coc_settings() {
   mkdir -p "$DEV_DIR"
-  # Mirror what the vim-lsp client sends (autoload/nuwiki/lsp.vim s:settings()):
-  # both `initializationOptions` (sent on initialize) and `settings.nuwiki`
-  # (served via workspace/configuration). Without these the server falls back
-  # to its own default wiki_root (~/vimwiki) instead of $WIKI_DIR, so it can't
-  # resolve links (e.g. Notes shows as broken) or create-on-follow correctly.
-  # toc_header_level / html_header_numbering are set to non-defaults so the
-  # dev wiki exercises them: :NuwikiTOC writes `== Contents ==` (level 2) and
-  # HTML export numbers headings. coc reads this file verbatim, so these are
-  # the values the server actually receives.
+  # No `languageserver.nuwiki` block here on purpose: this dogfoods nuwiki's
+  # *programmatic* coc registration (autoload/nuwiki/lsp.vim → coc#config). The
+  # plugin registers the server itself from g:nuwiki_* (set in the vimrc),
+  # exactly the "no hand-written coc-settings.json" flow real users get. Only
+  # the jump-command preference (the <CR>-opens-a-tab reproducer) lives here.
   cat > "$COC_SETTINGS" <<EOF
 {
-  "coc.preferences.jumpCommand": "$COC_JUMP",
-  "languageserver": {
-    "nuwiki": {
-      "command": "$REPO_ROOT/bin/nuwiki-ls",
-      "filetypes": ["vimwiki"],
-      "initializationOptions": {
-        "wiki_root": "$WIKI_DIR",
-        "file_extension": ".wiki",
-        "syntax": "vimwiki",
-        "log_level": "info",
-        "toc_header_level": 2,
-        "html_header_numbering": 2,
-        "html_header_numbering_sym": " -",
-        "diagnostic": { "link_severity": "warn" }
-      },
-      "settings": {
-        "nuwiki": {
-          "wiki_root": "$WIKI_DIR",
-          "file_extension": ".wiki",
-          "syntax": "vimwiki",
-          "log_level": "info",
-          "toc_header_level": 2,
-          "html_header_numbering": 2,
-          "html_header_numbering_sym": " -",
-          "diagnostic": { "link_severity": "warn" }
-        }
-      }
-    }
-  }
+  "coc.preferences.jumpCommand": "$COC_JUMP"
 }
 EOF
 }
@@ -146,6 +114,12 @@ let g:nuwiki_file_extension = '.wiki'
 let g:nuwiki_syntax       = 'vimwiki'
 let g:nuwiki_diary_rel_path = 'diary'
 let g:nuwiki_log_level    = 'info'
+" nuwiki auto-registers itself with coc from these globals (no coc-settings.json
+" languageserver block). Display settings exercise the global shorthand:
+" :NuwikiTOC writes `== Contents ==` (level 2) and HTML export numbers headings.
+let g:nuwiki_toc_header_level = 2
+let g:nuwiki_html_header_numbering = 2
+let g:nuwiki_html_header_numbering_sym = ' -'
 
 filetype plugin indent on
 syntax enable