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/sections/magic-prompts/MagicPromptsPage.tsx

@@ -172,6 +172,14 @@ const PROMPT_PAGE_MAP: Record<string, PromptPageConfig> = {
       { id: 'session.weigh.instructions', titleKey: 'settings.magicPrompts.page.block.instructions' },
     ],
   },
+  'session.explore': {
+    titleKey: 'settings.magicPrompts.page.group.sessionExplore.title',
+    descriptionKey: 'settings.magicPrompts.page.group.sessionExplore.description',
+    blocks: [
+      { id: 'session.explore.visible', titleKey: 'settings.magicPrompts.page.block.visiblePrompt' },
+      { id: 'session.explore.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

@@ -45,6 +45,7 @@ export const MagicPromptsSidebar: React.FC<MagicPromptsSidebarProps> = ({ onItem
       {
         groupKey: 'settings.magicPrompts.sidebar.group.session',
         items: [
+          { id: 'session.explore', titleKey: 'settings.magicPrompts.sidebar.item.sessionExplore' },
           { id: 'session.summary', titleKey: 'settings.magicPrompts.sidebar.item.sessionSummary' },
           { id: 'session.review', titleKey: 'settings.magicPrompts.sidebar.item.sessionWorkspaceReview' },
           { id: 'session.plan', titleKey: 'settings.magicPrompts.sidebar.item.sessionFeaturePlan' },