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

[baiwei0427]

Summary

Add a --prompt-file <path> flag to read the prompt from a file instead of from -p <text> 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

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