fix(vars): E121 in get_wikilocal no-index path; add regression test

s:resolve_index referenced an undefined `a:file` (a copy-paste leftover
from wiki_root_for, whose parameter is named a:file) inside the
buffer-owning-wiki loop. Any get_wikilocal() call with no explicit index
while a wiki buffer was current — i.e. the ftplugin BufRead path — hit
`E121: undefined variable a:file`. The local is `l:file`, and the
surrounding `!empty(l:file)` guard already covers emptiness, so the
clause was both wrong and redundant; removed it.

Add development/tests/test-vars-vim.{sh,vim} covering the shim:
per-wiki-index resolution, global-default fallback, out-of-range clamp,
the no-index owning-wiki path (direct guard for this E121), and the
single-wiki shorthand. Wired into CI. 14/14 pass.

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

development/tests/test-vars-vim.sh

@@ -0,0 +1,67 @@
+#!/usr/bin/env bash
+#
+# development/tests/test-vars-vim.sh — exercises the vimwiki compatibility
+# shim (autoload/vimwiki/vars.vim) that third-party plugins (vimwiki-sync,
+# vim-zettel) call via vimwiki#vars#get_wikilocal.
+#
+# Runs test-vars-vim.vim under headless Vim; that script writes PASS/FAIL
+# lines to $NUWIKI_VARS_OUT. Covers per-wiki-index resolution, global
+# fallback, out-of-range clamp, the no-index "owning wiki" path (the
+# BufRead path that previously raised E121), and the single-wiki shorthand.
+#
+# Exit code: 0 when every check passes, 1 on any FAIL or missing output.
+# Skips (exit 0) when vim is not installed.
+
+set -euo pipefail
+
+REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)"
+
+log() { printf '\033[1;34m[vars-vim]\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-vars-vim-test-XXXXXX)"
+trap 'rm -rf "$TMP"' EXIT
+
+DIR_A="$TMP/wiki_a"
+DIR_B="$TMP/wiki_b"
+mkdir -p "$DIR_A" "$DIR_B"
+printf '= Home =\n' > "$DIR_A/index.wiki"
+printf '= B =\n'    > "$DIR_B/index.wiki"
+
+OUT="$TMP/vars.out"
+
+VIMRC="$TMP/vimrc"
+cat > "$VIMRC" <<EOF
+set nocompatible
+let &runtimepath = '$REPO_ROOT' . ',' . &runtimepath
+EOF
+
+log 'running Vim vars-shim harness…'
+NUWIKI_VARS_OUT="$OUT" \
+NUWIKI_VARS_DIR_A="$DIR_A" \
+NUWIKI_VARS_DIR_B="$DIR_B" \
+  timeout 30 vim -e -s -u "$VIMRC" \
+    -c "source $REPO_ROOT/development/tests/test-vars-vim.vim" \
+    </dev/null >"$TMP/vim.log" 2>&1 || true
+
+if [[ ! -f "$OUT" ]]; then
+  echo 'vars harness produced no output' >&2
+  echo '--- vim log ---' >&2
+  cat "$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 'vars shim OK ✓'
+exit 0