agent: Instruct agents to request approval before creating threads (#61898)

# Objective

Instruct agents not to create persistent sibling threads without user
approval.

## Solution

Update the `create_thread` instructions to require an explicit request
or approval. Agents may still suggest creating a thread.

## 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 adheres to Zed’s UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

---

Release Notes:

- N/A

crates/agent/src/tools/create_thread_tool.rs

@@ -11,19 +11,19 @@ use crate::{AgentTool, SiblingThreadRequest, ThreadEnvironment, ToolCallEventStr
 
 /// Create a new agent thread that runs in parallel with this one.
 ///
-/// Use this to kick off separable pieces of work without interrupting the current
-/// conversation. The new thread appears in the agent sidebar just like a thread
-/// the user created themselves, and runs independently — you will NOT receive
-/// its output and you cannot interact with it afterwards. Use `spawn_agent`
-/// instead if you need the results back.
+/// Only use this after the user explicitly asks for or approves another thread.
+/// The new thread appears in the agent sidebar just like a thread the user created
+/// themselves, and runs independently — you will NOT receive its output and you
+/// cannot interact with it afterwards. Use `spawn_agent` instead if you need the
+/// results back.
 ///
 /// A successful call returns only the title, agent ID, and model used; there is
 /// currently no way to look up or control a sibling thread by session ID.
 ///
 /// ### When to use
-/// - The user asks you to start another thread, investigation, or exploration on the side.
-/// - You notice a separable task (refactor, bug fix, investigation) that shouldn't
-///   derail the current conversation but is worth pursuing.
+/// - The user explicitly asks you to create or start another thread.
+/// - You may suggest a new thread for a separable task, but call this tool only
+///   after the user explicitly agrees.
 ///
 /// ### Prompt design
 /// The new thread has no access to this conversation's history. Include in `prompt`