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>

docs/src/extensions/developing-extensions.md

@@ -1,11 +1,11 @@
 ---
 title: Developing Extensions
-description: "Create Zed extensions: languages, themes, debuggers, slash commands, and more."
+description: "Create Zed extensions: languages, themes, debuggers, and more."
 ---
 
 # Developing Extensions {#developing-extensions}
 
-Zed extensions are Git repositories containing an `extension.toml` manifest. They can provide languages, themes, debuggers, snippets, slash commands, and MCP servers.
+Zed extensions are Git repositories containing an `extension.toml` manifest. They can provide languages, themes, debuggers, snippets, and MCP servers.
 
 ## Extension Features {#extension-features}
 
@@ -16,7 +16,6 @@ Extensions can provide:
 - [Themes](./themes.md)
 - [Icon Themes](./icon-themes.md)
 - [Snippets](./snippets.md)
-- [Slash Commands](./slash-commands.md)
 - [MCP Servers](./mcp-extensions.md)
 
 ## Developing an Extension Locally