Have a way to specify per-invocation instructions.

### Describe the feature or problem you'd like to solve

I have prompts for various tasks, exploring code bases, writing C# code, or Azure SDK guideline aligned C# code, etc where the prompt is customized to the task. I want to be able to specify the prompt at startup for a single invocation.

### Proposed solution

Being able to specify a per-invocation set of instructions to tailor the specific interaction for the task it's going to be used for will allow the reuse of successful scenario based instructions while minimizing the per task customization required.

### Example prompts or workflows

I have files like:
arch.agent.md
azure_code.md
azure_csharp_code.md
azure_ts_code.md
code.md
email.md
explore-legacy.md
explore.md

that are all customized instructions for each type of task. I'd like to be able to specify them on start, something like
copilot --systemprompt @/email.md --instruction @/scratch/email_about_thing.md

### Additional context

_No response_