feat(chat): /summary slash command for non-destructive session summaries

Typing /summary (optionally with a topic hint) produces a structured
summary as a normal assistant message — without compacting or mutating
session history. Reuses the existing "Start new session from this answer"
button for hand-off.

Prompts are customizable in Settings → Magic Prompts under a new Session
group. Autocomplete shows an 'openchamber' badge to distinguish our
commands from OpenCode-built-in ones.

packages/ui/src/components/sections/magic-prompts/MagicPromptsSidebar.tsx

@@ -40,6 +40,12 @@ export const MagicPromptsSidebar: React.FC<MagicPromptsSidebarProps> = ({ onItem
           { id: 'plan.implement', title: 'Implement Plan' },
         ],
       },
+      {
+        group: 'Session',
+        items: [
+          { id: 'session.summary', title: 'Session Summary' },
+        ],
+      },
     ] as const;
   }, []);