Docs: Rewrite README for product-led structure

[FL4TLiN3]

Summary

• Restructure README around product components: Key Features → Quick Start → Deployment → Why Perstack
• Add Deployment section with lockfile (perstack install), Dockerfile, and programmatic API examples
• Replace prose-heavy "Why Perstack?" with problem-solution table
• Fix code examples to match actual API signatures (run() two-arg form, eventListener in options)
• Update Key Features bullets to outcome-based descriptions
• Add multi-provider support mention (8 LLM providers)

Test plan

• Verify all links resolve correctly
• Verify code examples match actual CLI and runtime API

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