fix(init): reject negative shard indexes

[zakazaka95]

Summary

• reject negative --shard_index values before connecting to L1
• add a CLI regression test for the invalid input

Why

--shard_index is parsed as a signed 64-bit value, but initialization later converts each index to uint64. A value such as -1 could therefore wrap to 18446744073709551615 instead of being rejected as invalid input. Validating immediately also avoids unnecessary RPC and configuration work.

The regression failed before the fix because initialization proceeded to the invalid RPC endpoint; after the fix it returns the exact non-negative validation error before any RPC work.

Testing

• go build -v ./cmd/...
• go test -v ./... -tags ci
• go vet -tags=ci ./...
• gofmt and git diff --check