fix(lint): fail loudly on a nonexistent REPO_ROOT in check-vale-style-sync.sh

A bad or stale REPO_ROOT argument fell through to the "neither copy
present" no-op guard and exited 0 — the exact "clean result can mean
nothing was checked" anti-pattern this PR spent multiple review rounds
eliminating elsewhere. That guard exists for a repo that legitimately has
no kyberforge plugin installed, not for a typo'd path.

Only the documented manual-invocation mode was affected: the shipped
pre-push hook always calls this script with zero args, which resolves via
`git rev-parse --show-toplevel` and is always valid inside a repo.

Added a regression test asserting a nonexistent REPO_ROOT exits non-zero.

Refs: #85

scripts/check-vale-style-sync.sh

@@ -10,11 +10,17 @@ set -euo pipefail
 # only one of the two. Run from repo root or pass REPO_ROOT as arg.
 
 REPO_ROOT="${1:-$(git rev-parse --show-toplevel 2>/dev/null || pwd)}"
+# A nonexistent REPO_ROOT must fail loudly, not fall through to the "neither
+# copy present" no-op below — that guard exists for a repo that legitimately
+# has no kyberforge plugin installed, not for a typo'd or stale path, and a
+# clean exit 0 here would read as "checked, in sync" when nothing ran at all.
+if [[ ! -d "$REPO_ROOT" ]]; then
+  echo "Vale style sync check failed: REPO_ROOT '$REPO_ROOT' is not a directory." >&2
+  exit 1
+fi
 # Absolutized because the glob probe below `cd`s into a scratch tree, where a
 # relative --config path would stop resolving.
-if [[ -d "$REPO_ROOT" ]]; then
-  REPO_ROOT="$(cd "$REPO_ROOT" && pwd)"
-fi
+REPO_ROOT="$(cd "$REPO_ROOT" && pwd)"
 FAIL=0
 
 err() { echo "  FAIL: $1" >&2; FAIL=$((FAIL + 1)); }