feat: Add SSH_CONFIG_INCLUDE_PATH option for read-only SSH configs

[clord]

Summary

Adds support for read-only SSH config files (e.g., managed by Nix Home Manager.)
New context option SSH_CONFIG_INCLUDE_PATH specifies an alternate file where DevPod
writes host entries. Then the main SSH config can use an Include directive to load
DevPod-managed entries

Motivation

Users with declaratively managed SSH configs (Nix, chezmoi, etc.) cannot have DevPod modify
~/.ssh/config directly since it's read-only or regenerated. This change allows DevPod to
write its host entries to a separate file that can be included.

Usage

Add an Include directive to your SSH config file ~/.ssh/config:

   Include ~/.ssh/devpod_hosts

Configure DevPod to use the include path:

devpod context set-options -o SSH_CONFIG_INCLUDE_PATH=~/.ssh/devpod_hosts

DevPod will now write workspace host entries to ~/.ssh/devpod_hosts instead of
~/.ssh/config.