[Certora] Add Certora verification for BabylonSpoke

[lilCertora]

Adds Certora specifications, harnesses and configurations for BabylonSpoke covering:

• Collateral restrictions: only the managed reserve can be collateral, users have at most one collateral and one debt reserve, and the managed reserve cannot be borrowed.
• Repayment bounds: the requested payment does not exceed debtToCover or the user's debt in the selected reserve.
• Collateral removal: the removed amount and the collateral balance reduction enforce the cap.
• Liquidation pricing: the removed collateral value does not exceed the payment value times liquidationBonus / 10000.
• Settlement: repayment targets the correct debt reserve, collateral removal targets the liquidator, and deficit reporting follows the remaining collateral and debt.

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Adds a GitHub Actions workflow for collateral restrictions, repayment bounds, collateral removal bounds and settlement accounting.