Graphite support

[atimmer]

When working in a team I heavily prefer Graphite over raw creating GitHub PRs.

This isn't fully fleshed out, but a start of how I envision it to work.

Vision
There is a per project setting that allows me to set Graphite as preferred for all intents an purposes. This makes t3code "stack-aware". The "Create PR" button now instead uses gt create and gt submit to create a PR through Graphite.

From an existing conversation I can click a "Build on top of this PR" that allows me to ask the LLM to build on top of the PR. When you click the "Create PR" button then will again call gt create and gt submit. So that will create a stack.

From an existing conversation I can also ask the LLM to modify the existing PR. That will use gt modify and gt submit.

Another thing I regularly do is ask the LLM to "fix any code reviews comments on this stack".

Probably the main thing I want is smaller PRs.