Add option for project-specific context files

Nobody has claimed this yet.

### General idea
I would love to see a feature similar to what Robitx/gp.nvim#50 has with their `:GpContext` command, namely being able to add a file like `.chatgpt.md` to the git root directory, the content of which would then be prepended to new chats - either as a system message, or actually prepended to the first user message.

### Usecase
I find myself re-typing the same context quite often, e.g.:
```
I am working on a django project. I have models A, B, and C. B has a foreign key to C. I also have a modelform AForm, BForm, CForm for each of those models.
```
before getting to my actual question.

Another usecase would be something like:
```
Only use python, typescript, tailwind CSS, and django templates (depending on what is necessary) to accomplish the following task:
```

Obviously this context is highly project dependant, but it is also very much a constant within each project.

### Proposed solution
Add two new options, e.g. `project_context_file` and `project_context_type`. If the `project_context_file` exists in the git root, set its content as the system message if `project_context_type == "system"` or prepend it to the first message of each new chat if `project_context_type == "prepend"` (or something along those lines).

Start by locating the plugin's configuration handling and the code that constructs messages for a new chat. Determine how the git root can be identified and how system messages differ from the first user message. Done means a project context file is optionally read and applied according to the selected mode, with both options documented and covered by tests if the project has relevant test coverage.

Written by the indexing model from the issue text.