docs: Add comprehensive security analysis for PR TauricResearch#281

Add organized security documentation addressing Gemini code review findings:

**Critical Fixes (PR281_CRITICAL_FIXES.md)**
- ChromaDB reset flag hardening (2 min fix)
- Path traversal prevention via input validation (10 min)
- CLI input validation at entry point (5 min)
Total time: 15-20 minutes

**Future Hardening Roadmap (FUTURE_HARDENING.md)**
- 20 security enhancements organized by priority (P0/P1/P2)
- 3-6 month phased implementation timeline
- Production readiness guidelines
- Compliance and enterprise considerations

**Key Findings**
- Gemini Issue TauricResearch#1 (Jupyter token): False positive - placeholder syntax
- Gemini Issue TauricResearch#2 (File uploads): Confirmed - wildcard accept with no validation
- Additional 15 architectural security issues documented for future work

**Organization**
- Clean docs/security/ structure (no root clutter)
- Quick reference tables and scannable formatting
- Actionable code snippets with before/after examples
- Risk matrix and effort estimates

Suitable for upstream contribution and production planning.

Author: claude