Add example pair for fail-fast required variables via ${var:?message} (rule 17)

[posidoni]

Context

reference/shell-standards.md rule 17
("Fail fast on a required variable with ${var:?message}") has no runnable
pair under examples/standards/. Follow the contract
in CONTRIBUTING.md.

Plan

1. Add examples/standards/13-fail-fast-required-var.good.sh:
   • Guard a required variable with : "${VAR:?message}" before using it in
     a path or command, so a missing/empty value aborts immediately with a
     clear message on stderr.
   • Self-contained (set the variable directly in the script so it runs to
     completion), no arguments, exits 0, clean under
     shellcheck --severity=warning and shfmt. Use mktemp -d + a trap
     for any filesystem work.
2. Add examples/standards/13-fail-fast-required-var.bad.sh:
   • Use the same variable unchecked (e.g. leave it defaulted to empty via
     : "${VAR:=}") so an empty value silently builds a broken path instead
     of failing.
   • Safe to run — keep every path inside the mktemp -d sandbox so an empty
     variable can never touch anything outside it.
     # expect-shellcheck: none — this is a style-guide-only pitfall,
     ShellCheck cannot detect a missing fail-fast check.
3. Add a row to the table in examples/standards/README.md.

Verify

task ci

must stay green.

A small ask

If you used an AI coding agent for this PR, feel free to say so — add a line
to the PR description like:

Authored by <your name/handle>, implemented using <model name>.

Either way is genuinely welcome here.