[Wave 6] Write Soroban contract deployment runbook (testnet to mainnet + rollback)

Problem

There's no written runbook for the testnet→mainnet contract deployment path. The only guidance is scattered across docs/SOROBAN_SETUP.md and the STELLAR_FEATURES_ENABLED flag description in the README, with no rollback procedure documented anywhere.

Tasks

• Write a deployment runbook covering: pre-deploy checklist, switching contract IDs from testnet to mainnet, flipping STELLAR_FEATURES_ENABLED safely, and post-deploy verification steps
• Document a rollback procedure: what to do if a deployed contract needs to be reverted or disabled without taking the whole app down
• Cross-link from xconfess-contracts/README.md and the main README's contributing section
• Include a copy-pasteable checklist (markdown checkboxes) for use during an actual deploy

Acceptance Criteria

• Runbook is a standalone doc under docs/
• A maintainer unfamiliar with the deploy process could follow it end-to-end without needing to ask questions
• Rollback section is explicit about what state the app is left in if STELLAR_FEATURES_ENABLED is flipped back to false mid-incident

Labels

documentation, contract, help wanted, Stellar Wave