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>

.github/CODEOWNERS.hold

@@ -32,9 +32,6 @@
 /crates/agent_ui/ @zed-industries/ai-team
 /crates/ai_onboarding/ @zed-industries/ai-team
 /crates/anthropic/ @zed-industries/ai-team
-/crates/assistant_slash_command/ @zed-industries/ai-team
-/crates/assistant_slash_commands/ @zed-industries/ai-team
-/crates/assistant_text_thread/ @zed-industries/ai-team
 /crates/bedrock/ @zed-industries/ai-team
 /crates/cloud_llm_client/ @zed-industries/ai-team
 /crates/codestral/ @zed-industries/ai-team