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/chat/CommandAutocomplete.tsx

@@ -164,6 +164,10 @@ export const CommandAutocomplete = React.forwardRef<CommandAutocompleteHandle, C
             ? [{ id: 'openchamber:debug', name: 'debug', source: 'openchamber' as const, description: t('chat.commandAutocomplete.command.debugDescription'), isOpenChamber: true }]
             : []
           ),
+          ...(canStartSessionCommand
+            ? [{ id: 'openchamber:weigh', name: 'weigh', source: 'openchamber' as const, description: t('chat.commandAutocomplete.command.weighDescription'), isOpenChamber: true }]
+            : []
+          ),
         ];
         const allCommands = [...builtInCommands, ...customCommands, ...skillCommands];
 
@@ -221,6 +225,10 @@ export const CommandAutocomplete = React.forwardRef<CommandAutocompleteHandle, C
             ? [{ id: 'openchamber:debug', name: 'debug', source: 'openchamber' as const, description: t('chat.commandAutocomplete.command.debugDescription'), isOpenChamber: true }]
             : []
           ),
+          ...(canStartSessionCommand
+            ? [{ id: 'openchamber:weigh', name: 'weigh', source: 'openchamber' as const, description: t('chat.commandAutocomplete.command.weighDescription'), isOpenChamber: true }]
+            : []
+          ),
         ];
 
         const filtered = (searchQuery