feat(codesec): implement CodeSec security analysis agent

[NadaBhm]

• Agent orchestrator with async pipeline (URL validation → clone → scan → score)
• Stack detection: language, framework, database, build tool, container info
• SAST scanner: Semgrep primary, Bandit fallback, OWASP Top 10 mapping
• Secrets scanner: regex patterns + GitLeaks fallback
• Dependency scanner: pip-audit + Safety + Trivy with deduplication
• Dockerfile scanner: Trivy + Hadolint + built-in regex checks
• SBOM generator: CycloneDX primary, Trivy fallback, manifest parsing
• Security scorer: 0-100 score, grade A-F, per-category breakdown
• Pydantic models for all results with full serialization
• 113 tests passing