Closes#13203.

coder templates push currently only reads the template tar fromstdin if the--directory flag is set to-. This is kind of awkward, and as seen in the attached issue, easy to miss. We can already check if the command is being piped to, so this is just a minor QOL improvement.

This breaks one valid albeit odd workflow, where a confirmation is piped into the command instead of using--yes/-y:

e.g:

echo 'yes' | coder templates push

would no longer work as expected.

Also updatesisTTY toisTTYIn, since the name suggests it could check eitherstdin orstdout, but it only checksstdin, andisTTYOut already exists.