feat: add advanced security agents for Python (PR #40)

Implements Agent C's v0.9.0 Advanced Security Agents:

**New Python Agents:**
- Attack Path Analyzer: BFS/DFS graph algorithms for attack path detection
- Supply Chain Analyzer: SBOM parsing (CycloneDX/SPDX) + vulnerability checking  
- Secrets Detector: 30+ patterns + entropy-based secret detection

**Integration:**
- Registered 3 agents in orchestrator with vantage support
- OPA policies for each agent type
- 39 comprehensive tests with pytest

**Reporting:**
- Interactive Findings section in HTML reports
- Session metadata and command transcripts
- Source badges for interactive discoveries

All main CI tests passing (Linux + macOS).

Co-authored-by: Agent C <agent-c@v0.9.0>