fix(process): #303 H10 — anchor is_pos to the leading marker (monotonic hardening)

Final re-review returned SHIP-IT with one contrived, pre-existing residual: a
line starting `Review-verdict: BLOCKED …` that ALSO contains a second literal
`review-verdict: mergeable` substring later on the same line read as positive.
Anchor the is_pos check to line-start so only the line's OWN leading verdict
word counts. Safe-by-construction: anchoring a positive matcher can only REDUCE
the allow-surface, so it cannot introduce a false-open (the dangerous
direction); the 21-case regression confirms no false-deny (all still
allow/deny/ask as before) + a new B6 case for this exact vector. No further
review round needed — the change is monotonic. shellcheck clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.claude/hooks/pretooluse-merge-consent.sh

@@ -133,7 +133,9 @@ while IFS= read -r line; do
   ref=$(printf '%s' "$line" | grep -ioE '@[[:space:]]*[0-9a-f]{7,40}' | head -1 \
         | grep -oiE '[0-9a-f]{7,40}' | tr 'A-F' 'a-f' || true)
   is_pos=0
-  if printf '%s' "$line" | grep -iqE 'review-verdict:[[:space:]]*(mergeable|approved|lgtm)'; then is_pos=1; fi
+  # Positive iff the line's OWN leading verdict word (right after the line-start marker) is positive —
+  # anchored so a second, later `review-verdict: mergeable` substring on a BLOCKED line can't flip it.
+  if printf '%s' "$line" | grep -iqE '^[[:space:]]*review-verdict:[[:space:]]*(mergeable|approved|lgtm)'; then is_pos=1; fi
   [ -z "$ref" ] && continue   # marker present but no @<sha> -> falls through to the final ask
   case "$sha" in
     "$ref"*) if [ "$is_pos" = 1 ]; then head_pos=1; else head_neg=1; fi ;;