feat: add secret detection with Secretlint (v0.8.0)

Add critical security feature to detect 350+ types of hardcoded secrets
using industry-standard Secretlint library.

Features:
- Detect AWS keys, GitHub tokens, NPM tokens, SSH keys, API keys, etc.
- All secrets marked as CRITICAL severity
- Context-aware remediation suggestions per secret type
- New SecretDetector using @secretlint/node
- New SecretViolation value object (100% test coverage)
- CLI output with "🔐 Secrets" section
- Async pipeline support for secret detection

Tests:
- Added 47 new tests (566 total, 100% pass rate)
- Coverage: 93.3% statements, 83.74% branches
- SecretViolation: 23 tests, 100% coverage
- SecretDetector: 24 tests

Dependencies:
- @secretlint/node: 11.2.5
- @secretlint/core: 11.2.5
- @secretlint/types: 11.2.5
- @secretlint/secretlint-rule-preset-recommend: 11.2.5

packages/guardian/src/domain/services/ISecretDetector.ts

@@ -0,0 +1,34 @@
+import { SecretViolation } from "../value-objects/SecretViolation"
+
+/**
+ * Interface for detecting hardcoded secrets in source code
+ *
+ * Detects sensitive data like API keys, tokens, passwords, and credentials
+ * that should never be hardcoded in source code. Uses industry-standard
+ * Secretlint library for pattern matching.
+ *
+ * All detected secrets are marked as CRITICAL severity violations.
+ *
+ * @example
+ * ```typescript
+ * const detector: ISecretDetector = new SecretDetector()
+ * const violations = await detector.detectAll(
+ *     'const AWS_KEY = "AKIA1234567890ABCDEF"',
+ *     'src/config/aws.ts'
+ * )
+ *
+ * violations.forEach(v => {
+ *     console.log(v.getMessage()) // "Hardcoded AWS Access Key detected"
+ * })
+ * ```
+ */
+export interface ISecretDetector {
+    /**
+     * Detect all types of hardcoded secrets in the provided code
+     *
+     * @param code - Source code to analyze
+     * @param filePath - Path to the file being analyzed
+     * @returns Array of secret violations found
+     */
+    detectAll(code: string, filePath: string): Promise<SecretViolation[]>
+}