fix(cli): route warnings, errors, logs to stderr

[seroperson]

Closes #956

Summary

This PR standardizes the output logic: stdout only for valuable data, stderr for logs/warnings/errors etc. Currently existing inconsistency breaks piping flows, like gitt issues list --json | jq, as stdout may be polluted with info which isn't relates to the calculated result.

It's a very common pattern of working with stdout/stderr (see https://clig.dev):

Send output to stdout. The primary output for your command should go to stdout. Anything that is machine readable should also go to stdout—this is where piping sends things by default.
Send messaging to stderr. Log messages, errors, and so on should all be sent to stderr. This means that when commands are piped together, these messages are displayed to the user and not fed into the next command.

And used in a wide amount of existing tools (curl, gh, kubectl etc).

Before

• gitt issues list --json | jq fails because of polluted stdout

After

• gitt issues list --json | jq succeeds because stdout contains only JSON output, the rest is in stderr

Test Plan

I can figure something out if necessary, but I'm really unsure whether it should be tested. Tell me if I need to cover this somehow.