test: Add secret detection benchmark dataset and ground truth

Add comprehensive benchmark dataset with 32 documented secrets for testing
secret detection workflows (gitleaks, trufflehog, llm_secret_detection).

- Add test_projects/secret_detection_benchmark/ with 19 test files
- Add ground truth JSON with precise line-by-line secret mappings
- Update .gitignore with exceptions for benchmark files (not real secrets)

Dataset breakdown:
- 12 Easy secrets (standard patterns)
- 10 Medium secrets (obfuscated)
- 10 Hard secrets (well hidden)

test_projects/secret_detection_benchmark/src/Crypto.go

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"fmt"
+	"strings"
+)
+
+// HARD SECRET #29: Heredoc with unusual delimiter
+const ConfigTemplate = `
+SECRET_KEY=golang_heredoc_secret_999
+END_OF_CONFIG
+`
+
+// HARD SECRET #30: Secret with intentional typo corrected programmatically
+const API_KEY_TYPO = "strippe_sk_live_corrected_key"
+
+func CorrectTypo(s string) string {
+	return strings.Replace(s, "strippe", "stripe", 1)
+}
+
+func main() {
+	fmt.Println("Crypto utilities initialized")
+	correctedKey := CorrectTypo(API_KEY_TYPO)
+	fmt.Println("Key ready:", correctedKey[:10]+"...")
+}