Support Claude $ARGUMENTS variable

[dyamon]

In olimorris/codecompanion.nvim#2147 we have been discussing about spec-kit support and it seems like the prompts/commands generated by specify for the copilot profile make use of $ARGUMENTS (a Claude feature).

CodeCompanion slash-commands are slightly different from slash commands in VS Code and as such it is not straightforward to implement this.
I see two options at the moment:

1. We treat $ARGUMENTS similar to ${input:...} substitution variables, prompting the user for input when using the slash-command.
2. Ask the spec-kit guys to support codecompanion by providing slightly different templates that expect the "arguments" after the custom prompt.