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/MagicPromptsPage.tsx

@@ -123,6 +123,14 @@ const PROMPT_PAGE_MAP: Record<string, PromptPageConfig> = {
       { id: 'plan.implement.instructions', title: 'Instructions' },
     ],
   },
+  'session.summary': {
+    title: 'Session Summary',
+    description: 'Prompts used by the /summary slash command: visible user message + hidden instructions. Non-destructive — does not compact session history.',
+    blocks: [
+      { id: 'session.summary.visible', title: 'Visible Prompt' },
+      { id: 'session.summary.instructions', title: 'Instructions' },
+    ],
+  },
 };
 
 const hasOwn = (input: Record<string, string>, key: string) => Object.prototype.hasOwnProperty.call(input, key);

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;
   }, []);