feat(benchmarks): auto-generate Interpretation/Recommendation and add -report-only flag

generate_report.go:
- buildInterpretation: derives narrative from p99/p50 tail-latency ratio,
  per-action complexity trend (% increase vs discover baseline), concurrency
  scaling efficiency (GOMAXPROCS=1 vs 16), and cache warm/cold delta
- buildRecommendation: identifies the best throughput/cost GOMAXPROCS level
  from scaling efficiency and adds production sizing guidance

run_benchmarks.sh:
- Add -report-only <dir> flag: re-runs parse_results.go + generate_report.go
  against an existing results directory without rerunning benchmarks

REPORT_TEMPLATE.md:
- Replace manual placeholders with __INTERPRETATION__ and __RECOMMENDATION__
  markers filled by the generator

benchmarks/run_benchmarks.sh

@@ -20,7 +20,6 @@ set -euo pipefail
 
 SCRIPT_START=$(date +%s)
 REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
-RESULTS_DIR="$REPO_ROOT/benchmarks/results/$(date +%Y-%m-%d_%H-%M-%S)"
 BENCH_PKG="./benchmarks/e2e/..."
 BENCH_TIMEOUT="10m"
 BENCH_TIME_SERIAL="10s"
@@ -31,6 +30,38 @@ BENCH_COUNT=1             # benchstat uses the 3 serial files for stability
 ONIX_VERSION="$(git -C "$REPO_ROOT" describe --tags --abbrev=0 2>/dev/null || echo "dev")"
 REPORT_TEMPLATE="$REPO_ROOT/benchmarks/reports/REPORT_TEMPLATE.md"
 
+# ── -report-only <dir>: regenerate report from an existing results directory ──
+if [[ "${1:-}" == "-report-only" ]]; then
+  RESULTS_DIR="${2:-}"
+  if [[ -z "$RESULTS_DIR" ]]; then
+    echo "Usage: bash benchmarks/run_benchmarks.sh -report-only <results-dir>"
+    echo "Example: bash benchmarks/run_benchmarks.sh -report-only benchmarks/results/2026-04-09_10-30-00"
+    exit 1
+  fi
+  if [[ ! -d "$RESULTS_DIR" ]]; then
+    echo "ERROR: results directory not found: $RESULTS_DIR"
+    exit 1
+  fi
+  echo "=== Regenerating report from existing results ==="
+  echo "Results dir : $RESULTS_DIR"
+  echo ""
+  cd "$REPO_ROOT"
+  echo "Parsing results to CSV..."
+  go run "$REPO_ROOT/benchmarks/tools/parse_results.go" \
+    -dir="$RESULTS_DIR" -out="$RESULTS_DIR" 2>&1 || true
+  echo ""
+  echo "Generating benchmark report..."
+  go run "$REPO_ROOT/benchmarks/tools/generate_report.go" \
+    -dir="$RESULTS_DIR" \
+    -template="$REPORT_TEMPLATE" \
+    -version="$ONIX_VERSION"
+  echo ""
+  echo "Done. Report written to: $RESULTS_DIR/BENCHMARK_REPORT.md"
+  exit 0
+fi
+
+RESULTS_DIR="$REPO_ROOT/benchmarks/results/$(date +%Y-%m-%d_%H-%M-%S)"
+
 cd "$REPO_ROOT"
 
 # ── benchstat is declared as a go tool in go.mod; no separate install needed ──