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/shared/constants/index.ts

@@ -86,6 +86,7 @@ export const SEVERITY_ORDER: Record<SeverityLevel, number> = {
  * Violation type to severity mapping
  */
 export const VIOLATION_SEVERITY_MAP = {
+    SECRET_EXPOSURE: SEVERITY_LEVELS.CRITICAL,
     CIRCULAR_DEPENDENCY: SEVERITY_LEVELS.CRITICAL,
     REPOSITORY_PATTERN: SEVERITY_LEVELS.CRITICAL,
     AGGREGATE_BOUNDARY: SEVERITY_LEVELS.CRITICAL,