feat(tbtc): add qc_v1 signer handoff

[mswilkison]

Summary

• add a real qc_v1 signer path in keep-core that produces signer handoff bundles
• build and sign the canonical unsigned migration spend, then return a typed handoff instead of a final artifact
• cover the new handoff path with direct pkg/tbtc tests

Testing

• go test ./pkg/tbtc -run focused covenant signer tests
• go test ./pkg/bitcoin ./pkg/covenantsigner ./config ./cmd