dev: exercise toc_header_level + html numbering in the dev launchers

The start-* dev harnesses generated a minimal server config (wiki_root,
file_extension, syntax, log_level, diagnostic) with no display settings,
so :NuwikiTOC always produced `= Contents =` (level 1) regardless of code
changes — there was no way to test toc_header_level/html numbering through
the harness. The coc launcher is worst: coc reads coc-settings.json
verbatim, bypassing the Vim client's config translation entirely.

Set toc_header_level=2, html_header_numbering=2, html_header_numbering_sym
in all three launchers:
- start-vim-coc.sh: into the generated coc-settings.json (init + settings).
- start-vim.sh: as g:nuwiki_* globals (folded into the wiki via the global
  shorthand).
- start-nvim.sh: in the setup() table.

Now the dev wiki demonstrates the settings: :NuwikiTOC writes
`== Contents ==` and HTML export numbers headings.

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

development/start-vim-coc.sh

@@ -77,6 +77,10 @@ write_coc_settings() {
   # (served via workspace/configuration). Without these the server falls back
   # to its own default wiki_root (~/vimwiki) instead of $WIKI_DIR, so it can't
   # resolve links (e.g. Notes shows as broken) or create-on-follow correctly.
+  # toc_header_level / html_header_numbering are set to non-defaults so the
+  # dev wiki exercises them: :NuwikiTOC writes `== Contents ==` (level 2) and
+  # HTML export numbers headings. coc reads this file verbatim, so these are
+  # the values the server actually receives.
   cat > "$COC_SETTINGS" <<EOF
 {
   "coc.preferences.jumpCommand": "$COC_JUMP",
@@ -89,6 +93,9 @@ write_coc_settings() {
         "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": {
@@ -97,6 +104,9 @@ write_coc_settings() {
           "file_extension": ".wiki",
           "syntax": "vimwiki",
           "log_level": "info",
+          "toc_header_level": 2,
+          "html_header_numbering": 2,
+          "html_header_numbering_sym": " -",
           "diagnostic": { "link_severity": "warn" }
         }
       }