feat(ui): context- and state-aware bottom keybinding strip (#27)

Per ADR-0051 (closing issue #27): the bottom status line is now a
keystrokes-only, state-selected strip. A pure status_bar_bindings()
picks the binding set by priority (first match wins):

  sidebar focus   → Ctrl-O next pane · ↑↓/PgUp/PgDn scroll · Esc input
  completion live → Tab/Shift-Tab cycle · Esc cancel · Enter run
  history nav     → ↑↓ browse · Esc clear · Enter run
  editing         → Esc clear · Ctrl-A/E home/end · Ctrl-W del word · Enter run
  default (empty) → Ctrl-O sidebar · Tab complete · ↑ history · Enter run

The editing state surfaces the #29 readline keys (ADR-0049's deferred
advertisement). Typed-command words (mode advanced/simple, the ':'
one-shot) and Ctrl-C quit leave the strip; simple mode's empty-input
hint gains a '`mode advanced` for SQL' pointer (advanced mode shows
none — a switcher knows the way back, and help covers it).

Mechanism: status_bar_bindings + a thin renderer (unit-testable);
App::is_browsing_history() exposes the private history_cursor; the mode
pointer lives in resolve_hint_lines. Catalog: 12 new shortcut labels +
panel.hint_mode_advanced (en-US.yaml + keys.rs, validator 1:1), 5 dead
strip strings removed.

Forks user-chosen: editing state shows #29 keys; quit omitted; no
width-drop machinery (longest strip ~65 cols fits; a width-budget test
keeps it lean). Modal-aware strip is OOS (pre-existing). Tests: 9
Tier-1 unit (per-state key sets, width budget, mode pointer), 1 Tier-3
rewritten, 15 full-panel snapshots re-accepted (reviewed). 2467 pass /
0 fail / 0 skip, clippy clean.

tests/it/walking_skeleton.rs

@@ -223,21 +223,30 @@ fn typing_colon_in_simple_mode_flips_prompt_to_advanced() {
 }
 
 #[test]
-fn status_bar_lists_quit_and_submit_in_all_modes() {
+fn status_bar_is_keystroke_only_and_state_aware() {
+    // ADR-0051 (issue #27): the bottom strip is keystrokes-only and
+    // tracks the interaction state. Typed-command words (`:` advanced
+    // once, `mode advanced`/`mode simple` switch) and `Ctrl-C quit`
+    // leave the strip; mode discovery moves to the hint (locked by the
+    // ui.rs unit tests). This test exercises the real render path.
     let mut app = App::new();
     let theme = Theme::dark();
 
-    let simple = rendered_text(&mut app, &theme, 80, 24);
-    assert!(simple.contains("Enter"), "status bar lists Enter");
-    assert!(simple.contains("Ctrl-C"), "status bar lists Ctrl-C");
-    assert!(simple.contains("mode advanced"));
+    // Default (empty input): nav / complete / history / run keystrokes.
+    let default_view = rendered_text(&mut app, &theme, 80, 24);
+    assert!(default_view.contains("Ctrl-O sidebar"), "strip lists sidebar:\n{default_view}");
+    assert!(default_view.contains("Enter run"), "strip lists run:\n{default_view}");
+    assert!(!default_view.contains("Ctrl-C"), "quit dropped from the strip:\n{default_view}");
+    assert!(
+        !default_view.contains("advanced once"),
+        "`:` command word dropped from the strip:\n{default_view}",
+    );
 
-    type_str(&mut app, "mode advanced");
-    submit(&mut app);
-    let advanced = rendered_text(&mut app, &theme, 80, 24);
-    assert!(advanced.contains("Enter"));
-    assert!(advanced.contains("Ctrl-C"));
-    assert!(advanced.contains("mode simple"));
+    // Editing (input has text): the #29 readline edit keys appear.
+    type_str(&mut app, "create");
+    let editing = rendered_text(&mut app, &theme, 80, 24);
+    assert!(editing.contains("Esc clear"), "editing strip lists clear:\n{editing}");
+    assert!(editing.contains("Ctrl-W del word"), "editing strip lists del word:\n{editing}");
 }
 
 // ---------------------------------------------------------------