P1: Rewrite README to match tested product reality

[jusso-dev]

Goal

README must accurately describe what works now, what is mocked, how to run a clean install, and how Muster complements Kelpie and Tawny. No aspirational feature may be presented as complete.

Scope

• Product positioning and tagline.
• Approved current product screenshot and logo.
• Direct links to Kelpie, Tawny and Bower with ownership boundaries.
• Clean Docker quick start, homelab port 3004, credentials behavior and trusted origins.
• Codex subscription authentication and private auth volume.
• Task board and three starter agent missions.
• Explicit mock/real integration status.
• GHCR public amd64 image usage and immutable tags.
• Tested browsers, architecture, security boundaries, backup/restore and troubleshooting links.
• Explicit roadmap/known limitations section.

Acceptance criteria

• Every claimed working feature has a route, implementation and automated/manual verification.
• No demo credentials, Yuma identity, fabricated metrics or secret appears.
• Clean install and demo/screenshot mode are clearly separated.
• Kelpie/Tawny/Bower links are direct and explain authoritative ownership.
• Commands run as written on supported Intel Ubuntu Docker host.
• Screenshot reflects current UI and contains synthetic data only.

Verification

• Execute README quick start on fresh host/volume.
• Link checker and secret scan.
• Review claims against current routes/tests/connector status.