sidebar: Group threads sidebar settings (#64395)

## Objective

Group the threads sidebar settings together in JSON and the Settings
Editor, using consistent “Threads Sidebar” terminology.

This follows up on #63452 now that #62883 has added configurable default
width, and includes the auto-open setting added since then.

## Solution

- Group `position`, `default_width`, and `auto_open` under
`agent.threads_sidebar`, and display their controls together in
Settings.
- Automatically migrate `agent.sidebar_side`,
`agent.threads_sidebar_default_width`, and
`agent.threads_sidebar_auto_open` across supported settings scopes,
preserving existing nested values.
- Apply changes to the default width immediately, including when the
sidebar was manually resized, and clear the saved width override.
Unrelated settings changes preserve manual sizing.
- Document all three settings, including the previously missing position
entry.

## Testing

The following targeted checks passed on the local changes:

- `cargo test -p migrator nest_agent_threads_sidebar_settings` — 7 tests
passed.
- `cargo test -p sidebar -p workspace -p agent_settings -p gpui_platform
--features gpui_platform/runtime_shaders --lib -- width
threads_sidebar_settings test_open_directory_in_existing_window` — 9
tests passed.
- `cargo fmt --all` completed successfully.
- `git diff --check` passed.
- From `docs`: `npx prettier --check src/reference/all-settings.md
src/ai/parallel-agents.md` passed.

Coverage includes migration precedence and supported scopes, settings
defaults and width limits, manual sizing and resets, persisted widths,
and auto-open behavior.

Full application tests and manual UI testing were not run. Clippy was
not rerun after the final terminology changes.

## 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)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Improved threads sidebar settings by grouping position, default width,
and auto-open together, and applying default-width changes even after
manual resizing.

docs/src/ai/parallel-agents.md

@@ -19,9 +19,11 @@ Terminal Threads also appear as entries in the sidebar alongside agent threads,
 
 To focus the sidebar without toggling it, use {#kb multi_workspace::FocusWorkspaceSidebar}. To search your threads, press {#kb agents_sidebar::FocusSidebarFilter} while the sidebar is focused.
 
-To change the sidebar’s default width, open the Settings Editor and search for “Threads Sidebar Default Width”. See [`agent.threads_sidebar_default_width`](../reference/all-settings.md#agent-threads-sidebar-default-width) for values and a configuration example.
+To change the sidebar’s position, open the Settings Editor and search for “Threads Sidebar Position”, which writes `agent.threads_sidebar.position`. To change its default width, search for “Threads Sidebar Default Width”. See [`agent.threads_sidebar.default_width`](../reference/all-settings.md#agent-threads-sidebar-default-width) for values and a configuration example.
 
-Drag the divider to override the configured width. Double-click it to return to the setting and follow subsequent settings changes.
+Drag the divider to resize the sidebar. Changing the default width updates even a manually resized sidebar. Double-click the divider to return to the configured width.
+
+The “Threads Sidebar Auto Open” setting controls whether opening a folder in an existing window opens the sidebar. Its JSON key is `agent.threads_sidebar.auto_open`.
 
 ### Switching Threads {#switching-threads}
 

docs/src/reference/all-settings.md

@@ -121,10 +121,32 @@ To use a fixed reset width, disable flexible sizing in the Settings Editor. Or a
 
 See [Agent Panel visual customization](../visual-customization.md#agent-panel) for other panel appearance settings.
 
+### Threads Sidebar Position {#agent-threads-sidebar-position}
+
+- Description: Which side of the window displays the [Threads Sidebar](../ai/parallel-agents.md#threads-sidebar).
+- Setting: `agent.threads_sidebar.position`
+- Default: `"left"`
+
+**Options**
+
+`"left"` or `"right"`.
+
+Open the Settings Editor and search for “Threads Sidebar Position”. Or add this to your `settings.json`:
+
+```json [settings]
+{
+  "agent": {
+    "threads_sidebar": {
+      "position": "right"
+    }
+  }
+}
+```
+
 ### Threads Sidebar Default Width {#agent-threads-sidebar-default-width}
 
 - Description: Default width in pixels of the [Threads Sidebar](../ai/parallel-agents.md#threads-sidebar).
-- Setting: `agent.threads_sidebar_default_width`
+- Setting: `agent.threads_sidebar.default_width`
 - Default: `300`
 
 **Options**
@@ -136,19 +158,21 @@ Open the Settings Editor and search for “Threads Sidebar Default Width”. Or
 ```json [settings]
 {
   "agent": {
-    "threads_sidebar_default_width": 360
+    "threads_sidebar": {
+      "default_width": 360
+    }
   }
 }
 ```
 
-If you haven't manually resized the sidebar, its width follows changes to this setting immediately. A manually resized width takes precedence until you double-click the divider to reset it. After resetting, the sidebar follows this setting again.
+Changing this setting immediately updates the sidebar width, even if you previously resized it manually. You can also double-click the divider to reset the sidebar to the configured width.
 
 Widths saved by older versions of Zed are preserved if they differ from the previous default of 300 pixels. A saved width of 300 pixels uses this setting instead.
 
 ### Threads Sidebar Auto Open {#agent-threads-sidebar-auto-open}
 
 - Description: Whether opening a folder in an existing window automatically opens the [Threads Sidebar](../ai/parallel-agents.md#threads-sidebar).
-- Setting: `agent.threads_sidebar_auto_open`
+- Setting: `agent.threads_sidebar.auto_open`
 - Default: `true`
 
 **Options**
@@ -162,7 +186,9 @@ Open the Settings Editor and search for “Threads Sidebar Auto Open”. Or add
 ```json [settings]
 {
   "agent": {
-    "threads_sidebar_auto_open": false
+    "threads_sidebar": {
+      "auto_open": false
+    }
   }
 }
 ```