## Objective

The Threads Sidebar opens at a hard-coded 300px and can be resized only
by dragging. `agent.default_width` controls the Agent Panel rather than
the sidebar, leaving no way to configure the sidebar’s preferred width.

## Solution

Add `agent.threads_sidebar_default_width` for the width used when no
user-chosen sidebar width is stored. The value is clamped to the
existing 200–800px drag range. Existing, unresized sidebars follow
settings changes immediately. Double-clicking the divider resets the
sidebar to the configured width and resumes following subsequent
settings changes.

The sidebar previously saved its width whenever any sidebar state
changed, not only when the divider was dragged. `SerializedSidebar` now
records `width_set_by_user`, so settings-driven widths are no longer
persisted. Manually resized widths take precedence until the divider is
reset.

## Testing

- `test_threads_sidebar_default_width` in
`crates/agent_settings/src/agent_settings.rs` covers the default, a user
override, fallback for `null` and an unset field, and preservation of
the Agent Panel width and sidebar position.
- Five width-focused tests in `crates/sidebar/src/sidebar_tests.rs`
cover reset behavior, live settings updates, clamping, legacy-width
restoration, and persistence. They also check that manually resized
widths resist settings changes and that resetting resumes following the
setting.
- To check by hand: set `"agent": { "threads_sidebar_default_width": 360
}`, reset the divider, and confirm the sidebar is 360px wide. Change the
setting and confirm the width updates immediately. Drag it wider,
restart, and confirm the dragged width is kept. Double-click the divider
and confirm it returns to the configured width. Set the value to `5000`
and confirm the untouched sidebar clamps to 800px.

## 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:

- Added `agent.threads_sidebar_default_width` for setting the width of
the Threads Sidebar.