chore: initialize Soroban accountability vault contract workspace

[1nonlypiece]

Initializes a Soroban Rust contract workspace under contracts/ for Disciplr accountability vaults/staking, to ground the contract issue backlog.

What this adds

• contracts/ Cargo workspace (soroban-sdk = "23", #![no_std])
• contracts/accountability_vault contract implementing the vault lifecycle: create_vault, stake, check_in, slash_on_miss, claim, withdraw, plus a read-only get_vault
• VaultStatus enum mirroring PersistedVault.status in src/types/vaults.ts; emitted events align with topics consumed by src/services/eventParser.ts
• 6 unit tests in src/test.rs (create/stake, check-in+claim, slash-on-miss, withdraw, and two failure cases) — all passing under cargo test
• contracts/README.md documenting the lifecycle and backend integration points

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