fix(lint): stop SIGPIPE from faking missing sections (#710)

`grep -q` exits at its first match without draining stdin, killing the
piping `echo` with SIGPIPE. Under `set -o pipefail` that 141 becomes the
pipeline's status, which is indistinguishable from "no match" — so a
section that is present gets reported missing. The race only surfaces on
bodies large enough that `echo` is still writing when `grep` bails, which
made the warning set differ between identical runs (full repo: 106/87/90
warnings across three runs; now a stable 59).

Feed both checks from a herestring so there is no writer to signal.

Co-authored-by: Jaak Vaher <jaak.vaher@cyber.ee>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

scripts/lint-agents.sh

@@ -91,7 +91,7 @@ lint_file() {
 
   # 2. Check required frontmatter fields
   for field in "${REQUIRED_FRONTMATTER[@]}"; do
-    if ! echo "$frontmatter" | grep -qE "^${field}:"; then
+    if ! grep -qE -- "^${field}:" <<<"$frontmatter"; then
       echo "ERROR $file: missing frontmatter field '${field}'"
       errors=$((errors + 1))
     fi
@@ -101,8 +101,12 @@ lint_file() {
   local body
   body=$(awk 'BEGIN{n=0} /^---$/{n++; next} n>=2{print}' "$file")
 
+  # Feed grep from a herestring, not a pipe: `grep -q` exits at the first match
+  # without draining its input, which kills a piping `echo` with SIGPIPE. Under
+  # `set -o pipefail` that 141 becomes the pipeline's status and is indistinguishable
+  # from "no match", so a large body raced its way to a spurious WARN.
   for section in "${RECOMMENDED_SECTIONS[@]}"; do
-    if ! echo "$body" | grep -qi "$section"; then
+    if ! grep -qi -- "$section" <<<"$body"; then
       echo "WARN  $file: missing recommended section '${section}'"
       warnings=$((warnings + 1))
     fi