docs: refresh README with real graph gallery

[forhappy]

Summary

• Rework the README into a polished, technical product introduction.
• Keep a real screenshot gallery with four Compass exports from public repositories.
• Restore detailed onboarding, install and upgrade behavior, VS Code navigation, assistant orientation, query pagination, MCP integration, Program IR evidence, history reports, CompassQL, capability boundaries, and documentation routing.
• Preserve the local-first, bounded, evidence-oriented technical positioning.

Screenshot sources

• BurntSushi/ripgrep at 435f59f
• CLIUtils/CLI11 at 60492bdd

Validation

• git diff --check
• Verified 47 local README link and asset targets
• Verified four 1600x1000 PNG gallery assets
• README now contains 2,861 words versus 2,417 in the original
• No Rust or JavaScript tests run because this change only affects documentation and images.