Docs: Rewrite README for outcome-first structure

[FL4TLiN3]

Summary

• Rewrote README to lead with concrete outcome (two CLI commands) instead of architecture
• Replaced Key Features / Quick Start / Deployment / FAQ with punch-first structure: tagline → code → Under the Hood → Go Deeper → Why Perstack → Community
• Added "Coming from other tools?" competitive positioning table
• Added TOML example showing what create-expert actually generates
• Added "Join the Community" section

Closes #484

Test plan

• Verify README renders correctly on GitHub
• Verify all links point to valid pages
• Confirm TOML example is syntactically valid

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