Update README for v2: MCP server, PostgreSQL, architecture diagram

Comprehensive rewrite documenting BloxCue v2 features:
- Architecture diagram showing unified search pipeline
- MCP server section with 6 tools and inject_context example
- PostgreSQL integration guide with safety guarantees
- Configuration reference table for all env vars
- v1 vs v2 comparison in FAQ
- Expanded troubleshooting for MCP and PG
- Updated roadmap with completed v2 milestones

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: “bokiko”