sidebar: Group thread sidebar settings and add default width

[joshkent94]

Objective

The threads sidebar position is currently a standalone agent.sidebar_side setting, and its width is fixed at 300px. Unlike dock panels, it has no configurable default width that updates the live sidebar.

Solution

• Group the sidebar settings under agent.thread_sidebar, migrate sidebar_side to position across settings scopes, and expose position and default_width in Settings.
• Use the configured width when creating or resetting the sidebar, update it when the setting changes, and persist the updated width when a sidebar is registered.

Testing

• cargo test -p migrator nest_agent_sidebar_side_under_thread_sidebar_position
• cargo test -p agent_settings test_thread_sidebar_settings
• cargo test -p sidebar -p gpui_platform --features gpui_platform/runtime_shaders thread_sidebar_default_width
• cargo test -p sidebar -p gpui_platform --features gpui_platform/runtime_shaders serialization_round_trip
• cargo fmt --all -- --check
• ./script/clippy -p agent -p agent_settings -p agent_ui -p migrator -p settings_content -p settings_ui -p sidebar -p workspace -p gpui_platform --features gpui_platform/runtime_shaders
• git diff --check

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content adheres to Zed's UI standards (UX/UI and icon guidelines)
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Recording

Screen.Recording.2026-08-30.at.18.50.27.mov

Release Notes:

• Added a configurable default width for the thread sidebar and grouped thread sidebar settings together.