feat: add ⌘L shortcut to focus message input with hint

Show a muted "⌘L to focus" hint in the top-right of the message input
when unfocused and empty on web. Registers Cmd+L (Mac) / Ctrl+L
(non-Mac) bindings for the existing message-input.focus action.

packages/app/src/keyboard/keyboard-shortcuts.ts

@@ -791,6 +791,32 @@ const SHORTCUT_BINDINGS: readonly ShortcutBinding[] = [
   },
 
   // --- Message input ---
+  {
+    id: "message-input-focus-cmd-l-mac",
+    action: "message-input.action",
+    combo: "Cmd+L",
+    when: { mac: true, commandCenter: false },
+    payload: { type: "message-input", kind: "focus" },
+    help: {
+      id: "focus-message-input",
+      section: "agent-input",
+      label: "Focus message input",
+      keys: ["mod", "L"],
+    },
+  },
+  {
+    id: "message-input-focus-ctrl-l-non-mac",
+    action: "message-input.action",
+    combo: "Ctrl+L",
+    when: { mac: false, commandCenter: false, terminal: false },
+    payload: { type: "message-input", kind: "focus" },
+    help: {
+      id: "focus-message-input",
+      section: "agent-input",
+      label: "Focus message input",
+      keys: ["mod", "L"],
+    },
+  },
   {
     id: "message-input-voice-toggle-cmd-shift-d-mac",
     action: "message-input.action",