feat: add /weigh approach-comparison command to draft welcome

Add /weigh as a draft chip and command for the moment before planning —
when you know what to build but not how. Its hidden prompt investigates
the code, then lays out 2-3 genuinely distinct approaches with trade-offs
(complexity, risk, blast radius, effort) and a clear recommendation,
without writing a plan or code.

Follows the established command pattern: visible + hidden magic prompts
wired into command autocomplete, the submit handler, draft preset chips,
and the Magic Prompts settings page, with i18n across all locales. Adds a
scales-3 icon to the sprite.

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

@@ -164,6 +164,14 @@ const PROMPT_PAGE_MAP: Record<string, PromptPageConfig> = {
       { id: 'session.debug.instructions', titleKey: 'settings.magicPrompts.page.block.instructions' },
     ],
   },
+  'session.weigh': {
+    titleKey: 'settings.magicPrompts.page.group.sessionWeigh.title',
+    descriptionKey: 'settings.magicPrompts.page.group.sessionWeigh.description',
+    blocks: [
+      { id: 'session.weigh.visible', titleKey: 'settings.magicPrompts.page.block.visiblePrompt' },
+      { id: 'session.weigh.instructions', titleKey: 'settings.magicPrompts.page.block.instructions' },
+    ],
+  },
   'session.fusion': {
     titleKey: 'settings.magicPrompts.page.group.sessionFusion.title',
     descriptionKey: 'settings.magicPrompts.page.group.sessionFusion.description',

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

@@ -50,6 +50,7 @@ export const MagicPromptsSidebar: React.FC<MagicPromptsSidebarProps> = ({ onItem
           { id: 'session.plan', titleKey: 'settings.magicPrompts.sidebar.item.sessionFeaturePlan' },
           { id: 'session.catchup', titleKey: 'settings.magicPrompts.sidebar.item.sessionCatchUp' },
           { id: 'session.debug', titleKey: 'settings.magicPrompts.sidebar.item.sessionDebug' },
+          { id: 'session.weigh', titleKey: 'settings.magicPrompts.sidebar.item.sessionWeigh' },
           { id: 'session.fusion', titleKey: 'settings.magicPrompts.sidebar.item.sessionFusion' },
         ],
       },