feat(benchmarks): add benchmark report, fix gitignore and README

- Add benchmarks/reports/REPORT_ONIX_v150.md — baseline report
  (Apple M5, darwin/arm64, Beckn v2.0.0, GOMAXPROCS=10)
- Gitignore benchmarks/results/ — runtime output from run_benchmarks.sh
- Update README: directory layout with reports/ vs results/, Reports
  section with workflow for adding new reports, fix benchstat invocation
  to use `go tool benchstat`
- Remove internal task marker from setup_test.go comment

.gitignore

@@ -131,6 +131,9 @@ dist
 .yarn/install-state.gz
 .pnp.*
 
+# Benchmark runtime output (raw go test output, logs, CSVs)
+benchmarks/results/
+
 # Ignore compiled shared object files
 *.so