Add a `--prompt-file <path>` flag (large prompts hit ARG_MAX; stdin is awkward)

## Summary

Add a `--prompt-file ` flag to read the prompt from a file instead of from `-p ` or stdin.

## Motivation

Today there are two ways to give Copilot CLI a prompt in non-interactive mode:

1. `-p "$(cat prompt.md)"` — convenient, but the whole prompt is passed on the command line, which is subject to `ARG_MAX` (~2 MB on typical Linux). For our use case (code-review bot that inlines the PR diff plus prior reviews) this is close enough to the limit to be a concern.
2. `< prompt.md` (stdin) — works, no size limit, but is awkward to thread through wrapper scripts that already use stdin for other purposes, and it's harder for new users to discover.

A first-class `--prompt-file` flag would:
- Have no size limit (just reads the file).
- Be self-documenting in scripts (`copilot --prompt-file ./review-prompt.md` vs `copilot -p "$(cat review-prompt.md)"`).
- Compose naturally with workflows that build prompts incrementally (instructions + diff + prior reviews) into a single file.

## Suggested behaviour

```bash
copilot --model gpt-5.5 --prompt-file ./prompt.md # equivalent to `-p "$(cat ./prompt.md)"` but unbounded
copilot --model gpt-5.5 --prompt-file - # explicit stdin (same as today's bare `<`)
```

Mutually exclusive with `-p`/`--prompt`. Error if the file doesn't exist or is empty.

## Why now

I just shipped a self-hosted GitHub Actions workflow that calls Copilot CLI three times per PR (two reviewers + synthesiser), each with a prompt that inlines the PR diff, repo instructions, and (for the synthesiser) the two prior reviews. The consensus prompt can approach several hundred KB; we use stdin today, which works but isn't obvious to anyone reading the workflow. `--prompt-file` would make the intent clear without a wrapper.

## Environment

- GitHub Copilot CLI: `1.0.49`
- Use case: non-interactive `-p`/stdin in GitHub Actions