Add support for using github copilot subscription

[rajeshg]

A lot of people have github copilot subscription. If we can use that subscription here, that would be great.

NOTE: opencode allows for proper usage of github copilot subscription. Could be a nice add for the "It’s OpenClaw and Claude Code, for finance" appeal of dexter.

[rajeshg]

This doesn't breach the terms of service of github copilot from what I see.

https://x.com/github/status/2011822451613712646