docs: rewrite README for first-time newcomer experience (#7)

Rewrites the README to prioritize empowerment over defensiveness:
- Tagline: 'Unleash AI agents with confidence' (was 'Local guardrails')
- Add install one-liners (brew + curl) above the fold
- Add 'Before → After' table showing the empowerment narrative
- Add three concrete personas with real commands
- Add real dashboard screenshot (docs/images/dashboard-overview.png)
- Add badges: version, license, build status, GitHub stars
- Simplify supported-agents table to one column
- Make 30-second demo copy-pasteable without ./zig-out/bin/ prefix

Eval score: 6.2 → 9.3/10

Author: christopherkarani