Square root by Newton's method as a CLI tool

Without calling any library square-root function, write a function that computes $\sqrt{a}$, and package it as a command-line program that reads the number from its arguments.

Newton iteration $x_{k+1}=\tfrac12\left(x_k+a/x_k\right)$, which converges quadratically. Reject negative input, return $0$ for $0$, start from a sensible guess (e.g. $a$ or $1$, or scale by powers of $4$ first) and stop when the relative change is below tolerance. The program should parse argv, validate that the argument is numeric and print a usage message on bad input.