Task 2.5: Extract cora-cli into lib + bin (for GitHub App backend reuse)

[ajianaz]

Split cora-cli into library (reusable review engine) + binary (CLI interface). This is needed so the GitHub App backend can import cora as a library instead of shelling out to the binary.

Checklist

• Create src/lib.rs with public exports
• Move CLI entry point to src/bin/cora.rs
• Export from lib: review::run(), scanner::scan_files(), types::*, config::*
• Ensure all existing cargo test tests still pass
• Ensure cargo build --release still produces same binary
• Verify cargo install still works
• Verify clippy and fmt still clean
• Update Cargo.toml if needed (lib vs bin sections)

Part Of: #118, #120