feat: turn Explore into a full /explore command

Promote the last plain-prompt draft chip into a proper /explore command so
all draft welcome presets are now commands. Its hidden prompt investigates
the repository and gives a structured orientation — big picture, main
modules, how they connect, conventions, and where to start — instead of a
file-by-file dump.

Follows the established pattern: visible + hidden magic prompts wired into
command autocomplete, the submit handler, the draft chip, and the Magic
Prompts settings page, with i18n across all locales. Drops the now-unused
explore prompt string.

packages/ui/src/components/chat/CommandAutocomplete.tsx

@@ -168,6 +168,10 @@ export const CommandAutocomplete = React.forwardRef<CommandAutocompleteHandle, C
             ? [{ id: 'openchamber:weigh', name: 'weigh', source: 'openchamber' as const, description: t('chat.commandAutocomplete.command.weighDescription'), isOpenChamber: true }]
             : []
           ),
+          ...(canStartSessionCommand
+            ? [{ id: 'openchamber:explore', name: 'explore', source: 'openchamber' as const, description: t('chat.commandAutocomplete.command.exploreDescription'), isOpenChamber: true }]
+            : []
+          ),
         ];
         const allCommands = [...builtInCommands, ...customCommands, ...skillCommands];
 
@@ -229,6 +233,10 @@ export const CommandAutocomplete = React.forwardRef<CommandAutocompleteHandle, C
             ? [{ id: 'openchamber:weigh', name: 'weigh', source: 'openchamber' as const, description: t('chat.commandAutocomplete.command.weighDescription'), isOpenChamber: true }]
             : []
           ),
+          ...(canStartSessionCommand
+            ? [{ id: 'openchamber:explore', name: 'explore', source: 'openchamber' as const, description: t('chat.commandAutocomplete.command.exploreDescription'), isOpenChamber: true }]
+            : []
+          ),
         ];
 
         const filtered = (searchQuery