Description

The sidebar (Work / Tasks / Agents / Context panel) width cannot be freely
adjusted by dragging the boundary between the main chat area and the sidebar.
The only way to change it is via /config sidebar_width <percent> which
requires typing a command and guessing the right percentage — there is no
interactive resize handle.

Current behavior

• Sidebar width is computed as a fixed pixel value from sidebar_width_percent
  (default 28%, range 10–50%), with a hard min of 24 cols and max of
  chat_width - 40.
• The layout uses Constraint::Length(sidebar_width) — a static length —
  so the split is fixed until the config is changed.
• No mouse drag, no resize handle, no keyboard shortcut to nudge the boundary.

Relevant code

• sidebar_width_for_chat_area():
  crates/tui/src/tui/ui.rs#L198-L208
• Layout constraint:
  crates/tui/src/tui/ui.rs#L6274
• Config default:
  crates/tui/src/settings.rs#L319

Suggested solution

Add a draggable resize handle (e.g., │ or ┃) between the chat area and
sidebar. Mouse down + drag should adjust sidebar_width_percent in real time
and persist the new value.

This is a common UX pattern in terminal apps (e.g., lazygit's side panel resize).