Stop slash menu printing disabled-reasons on enabled commands

The availability map computes a fall-through reason string for every
command whether or not it is disabled, and commandWithAvailability
copied it through unconditionally. Enabled menu items then displayed
false statements: '/fork ... Forking is already running.', '/copy ...
Requires transcript content.' on a session with a full transcript.
Carry the reason only when the command is actually disabled.

frontend-modern/src/components/AI/Chat/__tests__/assistantSlashCommands.test.ts

@@ -218,6 +218,23 @@ describe('assistantSlashCommands', () => {
     );
   });
 
+  it('drops the availability reason hint on enabled commands', () => {
+    const commands = filterAssistantSlashCommands('compact', undefined, {
+      availability: {
+        compact: {
+          disabled: false,
+          // The availability map always computes a fall-through reason; an
+          // enabled command must not display it as a false statement.
+          reason: 'Unavailable while another session action is running.',
+        },
+      },
+    });
+
+    expect(commands).toHaveLength(1);
+    expect(commands[0].disabled).toBe(false);
+    expect(commands[0].disabledReason).toBeUndefined();
+  });
+
   it('exposes canonical and alias tokens for the picker', () => {
     const help = filterAssistantSlashCommands('commands')[0];
     expect(getAssistantSlashCommandTokens(help)).toEqual(['help', 'commands']);

frontend-modern/src/components/AI/Chat/assistantSlashCommands.ts

@@ -248,7 +248,10 @@ const commandWithAvailability = (
   return {
     ...command,
     disabled: state.disabled,
-    disabledReason: state.reason,
+    // The availability map computes a fall-through reason even when the
+    // command is enabled; carrying it through made the menu show false
+    // statements like "Forking is already running." on enabled commands.
+    disabledReason: state.disabled ? state.reason : undefined,
   };
 };