[cDAC] add copilot prompt

[max-charlamb]

I found this prompt useful when using Copilot to interact with the cDAC, created while implementing #121418.

The prompt explains:

• The general goal of the cDAC
• Architecture
• Where relevant portions live (important paths)
• How to build and test

Adding the prompt to a copilot query enables a more natural conversation and workflow given the LLM 'understands' the cDAC project.

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[jkoritzinsky]

Can we set this up as a "GitHub custom agent" instead so it is easily usable from the Copilot agents panel as well as the Copilot CLI?

https://docs.github.com/en/copilot/concepts/agents/coding-agent/about-custom-agents

[steveisok]

Can we set this up as a "GitHub custom agent" instead so it is easily usable from the Copilot agents panel as well as the Copilot CLI?

https://docs.github.com/en/copilot/concepts/agents/coding-agent/about-custom-agents

What about having it in both? Is there something having it in the prompts folder covers that the custom agent doesn't?

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