[contracts] Deploy governance contract to testnet and register in LIVE_TESTNET.md

[CelestinaBeing]

Context

contracts/governance/ exists in the codebase but is not deployed to testnet. For the mainnet launch story, we need a live governance contract.

Work

• Complete any TODO items in contracts/governance/src/lib.rs
• Deploy to testnet using scripts/deploy-soroban-testnet.sh
• Add contract address and verification commands to LIVE_TESTNET.md
• Write a test that invokes propose, vote, and execute on the live testnet contract

Acceptance criteria

• Contract deployed and address in LIVE_TESTNET.md
• Live stellar contract invoke command documented
• Governance flow tested end-to-end on testnet