ux: warn when both a positional prompt and --prompt-file are provided

[practicalmind-dev]

Problem

When a user passes both an inline prompt and --prompt-file, the file silently
wins and the inline prompt is discarded with no warning:

assayer run "this is ignored" --prompt-file prompt.txt --models gpt-4o

There is no indication that "this is ignored" was dropped. This is confusing,
especially for users building wrapper scripts.

Location

assayer/cli/main.py, inside the run command:

if prompt_file:
    with open(prompt_file) as f:
        prompt_text = f.read().strip()
elif prompt:
    prompt_text = prompt

What to do

Add a warning to stderr when both are supplied:

if prompt_file:
    if prompt:
        click.echo(
            "Warning: --prompt-file takes precedence; the inline prompt is ignored.",
            err=True,
        )
    with open(prompt_file) as f:
        prompt_text = f.read().strip()

How to test

Add a test in tests/test_cli.py using CliRunner that passes both a prompt
argument and a --prompt-file, and asserts the warning appears in the output.
No mocking needed for this case, the validation runs before any API call.