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/cli/formatters/OutputFormatter.ts

@@ -9,6 +9,7 @@ import type {
     HardcodeViolation,
     NamingConventionViolation,
     RepositoryPatternViolation,
+    SecretViolation,
 } from "../../application/use-cases/AnalyzeProject"
 import { SEVERITY_DISPLAY_LABELS, SEVERITY_SECTION_HEADERS } from "../constants"
 import { ViolationGrouper } from "../groupers/ViolationGrouper"
@@ -177,6 +178,22 @@ export class OutputFormatter {
         console.log("")
     }
 
+    formatSecretViolation(sv: SecretViolation, index: number): void {
+        const location = `${sv.file}:${String(sv.line)}:${String(sv.column)}`
+        console.log(`${String(index + 1)}. ${location}`)
+        console.log(`   Severity: ${SEVERITY_LABELS[sv.severity]} ⚠️`)
+        console.log(`   Secret Type: ${sv.secretType}`)
+        console.log(`   ${sv.message}`)
+        console.log("   🔐 CRITICAL: Rotate this secret immediately!")
+        console.log("   💡 Suggestion:")
+        sv.suggestion.split("\n").forEach((line) => {
+            if (line.trim()) {
+                console.log(`      ${line}`)
+            }
+        })
+        console.log("")
+    }
+
     formatHardcodeViolation(hc: HardcodeViolation, index: number): void {
         console.log(`${String(index + 1)}. ${hc.file}:${String(hc.line)}:${String(hc.column)}`)
         console.log(`   Severity: ${SEVERITY_LABELS[hc.severity]}`)