feat: support --prompt-file across prompt-taking commands

[christophetd]

I needed to use this with a multiline, multi-paragraph prompt and it wasn't working, so I added a CLI flag to read the prompt from a file (or stdin).

The flag is now supported across all commands that take a prompt: generate, diagram, icon, pattern, story, and edit (for edit, the second positional [instruction] becomes optional). Pass - to read from stdin.

Example:

echo "isometric illustration of a coffee shop, watercolor style" | imagemage generate --prompt-file -
imagemage diagram --prompt-file ./diagram.txt
imagemage edit photo.png --prompt-file ./instruction.txt