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_prior_art.sh

@@ -75,7 +75,8 @@ fi
 # in the repo? (scribe_local_dups in scribe_defs.sh carries the why.)
 names=""
 if [ -n "$code" ]; then
-  names=$(printf '%s' "$code" | scribe_defs | sort -u | head -12) || names=""
+  # `|| true` inside: an early-exiting `head` must not void its own output (#4042).
+  names=$(printf '%s' "$code" | scribe_defs | sort -u | head -12 || true)
 fi
 
 local_lines=""
@@ -105,11 +106,11 @@ shapes="$names"
 if [ -z "$shapes" ] && [ -f "$file_path" ] && command -v tac >/dev/null 2>&1; then
   old_first=$(printf '%s' "$event" \
     | jq -r '.tool_input.old_string // .tool_input.old_str // empty' 2>/dev/null \
-    | grep -m1 -v '^[[:space:]]*$') || old_first=""
+    | grep -m1 -v '^[[:space:]]*$' || true)
   if [ -n "$old_first" ]; then
     ln=$(grep -nF -m1 -- "$old_first" "$file_path" 2>/dev/null | cut -d: -f1) || ln=""
     if [ -n "$ln" ]; then
-      shapes=$(head -n "$ln" "$file_path" | tac | scribe_defs | head -1) || shapes=""
+      shapes=$(head -n "$ln" "$file_path" | tac | scribe_defs | head -1 || true)
     fi
   fi
 fi