[Feature]: Support slash commands mid-prompt, not only at the start

[luisfelipesena]

Verify there is no existing issue for this

• I searched existing issues and this is not a duplicate

Description

Currently, slash commands (/undo, /new, /compact, custom commands) only trigger when typed at the very beginning of an empty prompt. The regex rawText.match(/^\/(\S*)$/) requires the entire input to be just the slash command.

This means users cannot type context first and then invoke a slash command, e.g.:

• explain the auth flow /compact — does nothing
• fix the bug in auth.ts /undo — does nothing

Expected: Typing / after a space anywhere in the prompt should open the slash command popover, just like @ mentions work mid-prompt today.

After selecting:

• Custom commands: replace the /trigger fragment with the expanded command, keep surrounding text
• Builtin commands: remove the /trigger fragment, keep surrounding text, execute the command

Related: the @ mention popover already works mid-prompt via textBeforeCursor.match(/@(\S*)$/). Slash commands should behave the same way.