[FEATURE]: ignored commands

[ariane-emory]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

For testing purposes, it would sometimes be useful to have custom slash commands whose output was ignored by the model.

Let's add an ignored property that can be used in the frontmatter of custom slash command Markdown files, whose default value is false. If set to true, the message generated by expanding the arguments/shell expansions in the command's Markdown file would be given the ignored property (and possibly the synthetic property, too, if appropriate? I'm not sure if that makes sense) to cause the model not to process the generated message. The user should still be able to see the resulting message, but the model should not process it.

An example of what such a command's Markdown file could look like follows:

---
description: List the files in the current directory.
agent: plan
ignored: true
---
Files in the current directory. 
!`ls -l .`