## 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.