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/lib/i18n/messages/zh-CN.ts

@@ -1420,7 +1420,6 @@ export const dict: Record<I18nKey, string> = {
   'chat.emptyState.draftTitle': 'What are we working on?',
   'chat.emptyState.draftTitleWithProject': 'What are we working on in {project}?',
   'chat.draftPresets.explore.label': 'Explore the codebase',
-  'chat.draftPresets.explore.prompt': 'Give me a high-level tour of this codebase — the architecture, the main modules, and how they fit together.',
   'chat.draftPresets.catchup.label': 'Catch me up',
   'chat.draftPresets.weigh.label': 'Weigh my options',
   'chat.draftPresets.plan.label': 'Start feature planning',
@@ -1490,6 +1489,7 @@ export const dict: Record<I18nKey, string> = {
   'chat.commandAutocomplete.command.catchUpDescription': '重新进入上下文：你之前在做什么、从哪里继续。',
   'chat.commandAutocomplete.command.debugDescription': '在提出修复方案前，引导式地排查 bug 的根本原因。',
   'chat.commandAutocomplete.command.weighDescription': '在动手前，权衡 2-3 种方案的利弊并给出推荐。',
+  'chat.commandAutocomplete.command.exploreDescription': '快速熟悉这个代码库：对架构和主要部分的概览。',
   'chat.commandAutocomplete.badge.skill': '技能',
   'chat.commandAutocomplete.badge.command': '命令',
   'chat.commandAutocomplete.badge.system': '系统',
@@ -1609,6 +1609,7 @@ export const dict: Record<I18nKey, string> = {
   'chat.chatInput.toast.catchUpFailed': '无法获取上下文',
   'chat.chatInput.toast.debugFailed': '无法开始调试',
   'chat.chatInput.toast.weighFailed': '无法权衡方案',
+  'chat.chatInput.toast.exploreFailed': '无法开始导览',
   'chat.chatInput.toast.attachmentsTooLarge': '附件过大，无法发送。请减少图片数量或大小。',
   'chat.chatInput.toast.sendAttachmentsFailed': '发送附件失败。请尝试更少文件或更小图片。',
   'chat.chatInput.toast.messageSendFailed': '消息发送失败，附件已恢复。',