feat(tbtc): implement self_v1 signer completion

[mswilkison]

Summary

• wire the covenant signer server to a real engine from tbtc instead of the passive stub
• add the first real self_v1 signer path that fetches the active outpoint, verifies the reserved migration destination and transaction plan, signs the canonical maturity spend, and returns READY with transactionHex + deterministic artifact hash
• extend the bitcoin transaction builder with explicit locktime/sequence/witness controls and add end-to-end signer coverage

Testing

• go test ./pkg/bitcoin ./pkg/covenantsigner
• go test ./pkg/tbtc -run TestCovenantSignerEngine_SubmitSelfV1Ready -v
• go test ./config ./cmd