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_after_write.sh

@@ -138,7 +138,8 @@ while IFS= read -r rel_path; do
     code=$(cat "$file_path" 2>/dev/null) || code=""
   fi
   [ -n "$code" ] || continue
-  names=$(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' "$code" | scribe_defs | sort -u | head -12 || true)
   # Nothing DEFINED in what was written (prose, data, a call-site edit) →
   # nothing to say; the arms are about shapes.
   [ -n "$names" ] || continue