fix(plugin): an early-exiting head no longer voids its own output under pipefail (#4042)

In a repo where 3,000 files define `slug`, scribe_local_dups printed
nothing; with 3 files it printed the duplicate line. Every hook runs under
`set -uo pipefail`, and `hits=$(git grep -l … | head -4) || hits=""` lost
head's four lines whenever git grep was still writing when head exited. That
is a SIGPIPE, the substitution fails, and the outer fallback wipes the result.
So the by-name duplicate arm went silent for exactly the most-duplicated
names. Found while fixing the same trap in the new Stop hook (#4041).

- The fallback moves inside the substitution, `$(… | head -N || true)`, at all
  five sites: scribe_defs.sh (local dups), scribe_prior_art.sh (names,
  old_first, shapes) and scribe_after_write.sh (names). A real upstream
  failure still yields empty output.
- check_plugin gains a known-bad pattern for the shape, so no hook can bring
  it back.
- Tests: the real function against a 3,000-file repo (past the pipe buffer),
  and the pattern shown to flag the old shape and pass the fix. Plugin
  version minted.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

plugin/hooks/scribe_defs.sh

@@ -114,7 +114,12 @@ scribe_local_dups() {
       *)   pat="(function|def|class|func|fun|fn|sub|struct|trait|interface|enum|object|protocol|type)[[:space:]]+${name}[^A-Za-z0-9_]|func[[:space:]]*\([^)]*\)[[:space:]]*${name}[[:space:]]*\(|(const|let)[[:space:]]+${name}[[:space:]]*=" ;;
     esac
     # -I skips binaries; :(exclude) drops the file being written.
-    hits=$(git -C "$root" grep -I -l -E -e "$pat" -- . ":(exclude)${rel}" 2>/dev/null | head -4) || hits=""
+    # `|| true` INSIDE the substitution, not `|| hits=""` outside it (#4042):
+    # under the hooks' `pipefail`, `head` exiting after four lines kills a
+    # git grep that is still writing, the pipeline reports SIGPIPE, and an
+    # outer fallback then wipes the four hits head already printed. The name
+    # most duplicated — the one this arm exists for — was the one it dropped.
+    hits=$(git -C "$root" grep -I -l -E -e "$pat" -- . ":(exclude)${rel}" 2>/dev/null | head -4 || true)
     [ -n "$hits" ] || continue
     count=$(printf '%s\n' "$hits" | grep -c . 2>/dev/null || echo 0)
     label=$([ "$kind" = css ] && printf '.%s' "$name" || printf '%s' "$name")