feat: re-platform agentic review onto executus + large-PR cost controls (#20)

Makes gadfly a consumer of executus (run.Executor compaction/bounding/budget/critic + fanout) and fixes the large-PR token burn in size-gated layers: paginated get_diff, downshift above GADFLY_HUGE_DIFF_BYTES, and a swarm-wide GADFLY_PR_BUDGET_SECS backstop. Small PRs untouched; advisory-only and the static binary preserved. Dogfood swarm reviewed it (6 models, 21 real findings graded + folded in).

Co-authored-by: Steve Dudenhoeffer <steve@stevedudenhoeffer.com>
Co-committed-by: Steve Dudenhoeffer <steve@stevedudenhoeffer.com>

scripts/run.sh

@@ -177,7 +177,7 @@ case "$PROVIDER" in
           GADFLY_SYSTEM_FILE="${SCRIPT_DIR}/system-prompt.txt" \
           GADFLY_TITLE="$TITLE" \
           GADFLY_BODY="$BODY" \
-          GADFLY_MAX_DIFF_CHARS="$MAX_DIFF_CHARS" \
+          GADFLY_MAX_DIFF_CHARS="${GADFLY_MAX_DIFF_CHARS:-$MAX_DIFF_CHARS}" \
           GADFLY_STATUS_FILE="${GADFLY_STATUS_FILE:-}" \
           GADFLY_FINDINGS_OUT="${GADFLY_FINDINGS_OUT:-}" \
           "$BIN" 2>"$ERR_FILE"
@@ -230,8 +230,12 @@ fi
 if [ "$CONSOLIDATE" = "1" ]; then
   say "done in ${DUR} (consolidated; no per-model comment)"
 else
-  COMMENT="$(printf '%s\n### 🪰 Gadfly review — `%s` (%s)\n\n%s\n\n<sub>Automated adversarial review by Gadfly. Advisory only — does not block merge. · ⏱️ reviewed in %s</sub>' \
-    "$MARKER" "$MODEL" "$MODEL_PROVIDER" "$REVIEW" "$DUR")"
+  # An optional one-line notice (e.g. entrypoint's huge-PR downshift advisory),
+  # shown under the header so a shallower review is self-explaining.
+  NOTICE_BLOCK=""
+  [ -n "${GADFLY_NOTICE:-}" ] && NOTICE_BLOCK="> ${GADFLY_NOTICE}"$'\n\n'
+  COMMENT="$(printf '%s\n### 🪰 Gadfly review — `%s` (%s)\n\n%s%s\n\n<sub>Automated adversarial review by Gadfly. Advisory only — does not block merge. · ⏱️ reviewed in %s</sub>' \
+    "$MARKER" "$MODEL" "$MODEL_PROVIDER" "$NOTICE_BLOCK" "$REVIEW" "$DUR")"
   upsert_comment "$COMMENT"
   say "done in ${DUR}"
 fi