Don't /drop read-only files added at launch

Commentary

• 0:01 We're going to update the /drop command to keep any read only files that were originally specified at launch.
• 0:10 We've added files that handle the main CLI and in-chat slash commands like /drop.
• 0:20 Let's explain the needed change to aider.
• 1:20 Ok, let's look at the code.
• 1:30 I'd prefer not to use "hasattr()", let's ask for improvements.
• 1:45 Let's try some manual testing.
• 2:10 Looks good. Let's check the existing test suite to ensure we didn't break anything.
• 2:19 Let's ask aider to add tests for this.
• 2:50 Tests look reasonable, we're done!