Make the number of parallel code-host requests configurable (git config stack.codeHostConcurrency)

## What I'd like

A way to control how many requests stack makes to the code host at the same time, set through git config like the other options — for example:

git config stack.codeHostConcurrency 2

## Why

Right now stack fires several code-host requests in parallel (it looks like 4 at a time). In my setup, that burst of simultaneous requests is too much and the calls start timing out, so stack fails partway through.

If I could set the number myself, I could find the sweet spot that works for me — maybe 3 or 2 is enough, and 1 as a worst case. Being able to tune it means stack finishes reliably instead of failing on the burst.

## How I imagine it working

The number already exists in the code (codeHostConcurrency, default 4) but isn't exposed. It could be read from git config the same way stack.trunks and stack.blockLink already are:

git config stack.codeHostConcurrency 2 # two requests at a time
git config stack.codeHostConcurrency 1 # one at a time
git config stack.codeHostConcurrency 4 # current default

If it's not set, keep the current default of 4 so nothing changes for existing users.

Search for codeHostConcurrency and the existing stack.trunks and stack.blockLink Git configuration handling. Add support for stack.codeHostConcurrency while preserving the default of 4 when unset, then verify that code-host requests use the configured limit and existing behavior remains unchanged by default.

Written by the indexing model from the issue text.