feat(config): vimwiki drop-in config + global per-wiki defaults

Two related config-ergonomics features for vimwiki migrants, sharing the
same scalar-global machinery.

1. Upstream g:vimwiki_* drop-in (Vim client). When nuwiki isn't configured
   natively, the Vim client reads g:vimwiki_list + g:vimwiki_* globals and
   translates them into nuwiki's schema: per-wiki path->root,
   path_html->html_path, template_*, css_name, auto_export/auto_toc,
   syntax, ext->file_extension, index, diary_*, name. g:nuwiki_* always
   wins. Lets a vimwiki user drop in nuwiki without rewriting config.

2. Global per-wiki defaults (both clients). A display/generation setting
   given once at the top level — g:nuwiki_<key> / setup({<key>=…}) /
   g:vimwiki_<key> — is folded into every wiki as a default; a per-wiki
   value overrides it (vimwiki's model). Covers toc_header(_level),
   toc_link_format, links_header(_level), tags_header(_level),
   html_header_numbering(_sym), links_space_char, list_margin, listsyms,
   listsym_rejected, and the auto_* toggles. Only values the user
   explicitly set fold — built-in defaults don't (added config.user
   tracking on the Lua side so a default toc_header_level=1 isn't pushed
   onto every wiki). User config tables are never mutated.

Both clients kept in lock-step (config-parity golden enforces identical
payloads). New harnesses test-vimwiki-compat-vim (18) and
test-global-shorthand (8), wired into CI. Server test
vimwiki_compat_payload_sets_toc_level_per_wiki. Docs in README +
known-issues.md (drop-in is currently Vim-only). Rust 573 passed, clippy
clean, all harnesses green.

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

development/tests/test-global-shorthand.sh

@@ -0,0 +1,54 @@
+#!/usr/bin/env bash
+#
+# development/tests/test-global-shorthand.sh — Neovim client global-shorthand
+# folding (vimwiki-style per-wiki defaults). Runs test-global-shorthand.lua
+# under headless Neovim; that script asserts the init_options payload folds a
+# top-level / g:nuwiki_* setting into every wiki, lets a per-wiki value
+# override it, and does NOT fold built-in defaults.
+#
+# Exit code: 0 when every check passes, 1 on any FAIL or missing output.
+# Skips (exit 0) when nvim is not installed.
+
+set -euo pipefail
+
+REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)"
+
+log() { printf '\033[1;34m[global-shorthand]\033[0m %s\n' "$*"; }
+
+if ! command -v nvim >/dev/null 2>&1; then
+  log 'nvim not installed — skipping'
+  exit 0
+fi
+
+TMP="$(mktemp -d -t nuwiki-gs-test-XXXXXX)"
+trap 'rm -rf "$TMP"' EXIT
+
+OUT="$TMP/gs.out"
+INIT="$TMP/init.lua"
+cat > "$INIT" <<EOF
+vim.opt.runtimepath:prepend('$REPO_ROOT')
+EOF
+
+log 'running Neovim global-shorthand harness…'
+NUWIKI_GS_OUT="$OUT" \
+  timeout 30 nvim --clean -u "$INIT" --headless \
+    -c "luafile $REPO_ROOT/development/tests/test-global-shorthand.lua" \
+    >"$TMP/nvim.log" 2>&1 || true
+
+if [[ ! -f "$OUT" ]]; then
+  echo 'global-shorthand harness produced no output' >&2
+  echo '--- nvim log ---' >&2
+  cat "$TMP/nvim.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 'global shorthand OK ✓'
+exit 0