feat: restructure README for SEO and branding

- Add 'What Can Local AI Actually Do?' section with HomeSec-Bench (131 tests)
- Move legacy Docker/CLI content (Applications 1-5, step-by-step HA guide) to docs/legacy-applications.md
- Replace detailed legacy content with compact <details> menu and summary table
- Merge duplicate FAQ, Architecture, Support, and Commercial sections
- Update overview: add 'AI camera' keyword, CCTV/IP/webcam mentions, benchmark reference
- Add Telegram/Discord/Slack messaging mention to overview
- Reduce README from 494 to ~200 lines while preserving all SEO-critical keywords

SEO Keywords preserved: camera(37), AI(58), VLM(12), detection(17), CCTV(2),
surveillance(2), facial recognition(3), RE-ID(5+3), security(6), benchmark(3)

Author: solderzzc