docs: improve README clarity and structure

[Cloriskitty]

Summary

• Unify positioning: Consistent "economic control and trust infrastructure" language throughout (was split between "execution layer" in meta and "trust infrastructure" in body)
• Add "Why OmniClaw" section: Two-paragraph problem statement so new visitors understand the value prop before the feature list
• Reorder Quick Start: Server → CLI → Code (was CLI → Server, which confused the flow)
• Fix clone URL: Corrected omniclaw/omniclaw → omnuron/omniclaw
• Remove .env duplication: Environment variables listed once instead of twice
• Unify editorial voice: Single consistent tone across the entire document

Test plan

• Verify README renders correctly on GitHub
• Check all Wiki links resolve
• Confirm clone URL works