Add `--include-private` to `just --list` and `just --summary`

Currently there is no way (that I know of) to get a complete list of recipes, including private. Would it be possible to add a flag like `--include-private` that displays them?

Start by locating the CLI handling for --list and --summary, then trace where private recipes are filtered from their output. Done means --include-private makes both commands include private recipes while the existing output remains unchanged without the flag.

Written by the indexing model from the issue text.