Remove text thread and slash command crates (#52757)

🫡

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Removed legacy Text Threads feature to help streamline the new agentic
workflows in Zed. Thanks to all of you who were enthusiastic Text Thread
users over the years ❤️!

---------

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>

crates/docs_preprocessor/src/main.rs

@@ -543,9 +543,9 @@ fn template_and_validate_json_snippets(book: &mut Book, errors: &mut HashSet<Pre
 /// This will return the action name unmodified.
 ///
 /// ```
-/// let action_as_str = "assistant::Assist";
+/// let action_as_str = "workspace::Save";
 /// let action_name = name_for_action(action_as_str);
-/// assert_eq!(action_name, "assistant::Assist");
+/// assert_eq!(action_name, "workspace::Save");
 /// ```
 ///
 /// This will return the action name with any trailing options removed.