feat: GitHub Copilot Provider

[GlenEder]

GitHub Copilot Provider

• Adds GitHub's copilot cli tool as a new provider to the list.
• Updated Readme to show default model for copilot
• Added test cases in matching style to other providers

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[sentry]

Bug: The GitHubCopilotProvider incorrectly expects the copilot CLI to write its response to a temp file, but the CLI outputs to stdout, resulting in empty responses.
_{Severity: CRITICAL}

Suggested Fix

Modify the provider's request handling. Either find a copilot CLI flag that directs output to the specified temporary file, or change the callback logic to write the captured stdout data into the temporary file before _retrieve_response is called.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: lua/99/providers.lua#L334-L343

Potential issue: The `GitHubCopilotProvider` is configured to read the AI's response
from a temporary file. However, it invokes the `copilot` CLI with the `--prompt` flag,
which, according to its documentation, directs the output to `stdout`. The system
captures this `stdout` but never writes it to the temporary file that the
`_retrieve_response` function attempts to read. As a result, the temporary file remains
empty, causing the provider to return an empty response and rendering it completely
non-functional.

_{Did we get this right? 👍 / 👎 to inform future reviews.}

[ThePrimeagen]

i already do not to provide any more providers. just create the thing yourself and pass it in as the provider override.

base provider is exposed