feat(ui): width-derived sidebar visibility — hide at <=90 cols (#21, ADR-0046 DB1)

The schema sidebar (the left Tables column) is now shown only when the
terminal is wider than 90 columns; at or below that it is hidden and
the output/input panels span the full width. This reclaims horizontal
space on narrow terminals — notably the 90-column screencasts, where
the sidebar added little and cost the output panel its width.

Visibility is a pure function of terminal width (sidebar_visible);
the Ctrl-O peek-reveal lands in Phase C. render() splits the layout
conditionally — full-width right column when the sidebar is hidden.

Snapshots/tests that rendered at 80 wide now reflect the hidden
sidebar; those whose intent IS the sidebar (populated_with_table, the
items-panel and drop-table integration checks) render at 110 so the
Tables list is actually exercised — one masked-intent integration
check (matched "Customers" in the output, not the panel) is corrected
the same way. New tests cover the width gate and the show/hide
boundary.

tests/it/walking_skeleton.rs

@@ -301,7 +301,8 @@ fn create_table_flow_updates_tables_list_and_structure_view() {
     assert_eq!(app.tables, vec!["Customers".to_string()]);
     assert_eq!(app.current_table, Some(desc));
 
-    let rendered = rendered_text(&mut app, &theme, 80, 24);
+    // Width > 90 so the sidebar (items panel) is shown (ADR-0046 DB1).
+    let rendered = rendered_text(&mut app, &theme, 110, 24);
     assert!(
         rendered.contains("Customers"),
         "items panel should list Customers:\n{rendered}"
@@ -397,7 +398,8 @@ fn drop_table_flow_clears_items_list() {
 
     assert!(app.tables.is_empty());
     assert!(app.current_table.is_none());
-    let rendered = rendered_text(&mut app, &Theme::dark(), 80, 24);
+    // Width > 90 so the (now-empty) sidebar is shown (ADR-0046 DB1).
+    let rendered = rendered_text(&mut app, &Theme::dark(), 110, 24);
     assert!(rendered.contains("(none yet)"));
     // ADR-0040: `drop table` is content-less, so the echo's ✓ marker
     // is the entire success signal (replacing `[ok] drop table …`).