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-TW.ts

@@ -1417,7 +1417,6 @@ 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',
@@ -1487,6 +1486,7 @@ export const dict: Record<I18nKey, string> = {
   'chat.commandAutocomplete.command.catchUpDescription': '重新進入上下文：你之前在做什麼、從哪裡繼續。',
   'chat.commandAutocomplete.command.debugDescription': '在提出修復方案前，引導式地排查 bug 的根本原因。',
   'chat.commandAutocomplete.command.weighDescription': '在動手前，權衡 2-3 種方案的利弊並給出推薦。',
+  'chat.commandAutocomplete.command.exploreDescription': '快速熟悉這個程式碼庫：對架構和主要部分的概覽。',
   'chat.commandAutocomplete.badge.skill': 'Skills',
   'chat.commandAutocomplete.badge.command': '命令',
   'chat.commandAutocomplete.badge.system': '系統',
@@ -1606,6 +1606,7 @@ export const dict: Record<I18nKey, string> = {
   'chat.chatInput.toast.catchUpFailed': '無法取得上下文',
   'chat.chatInput.toast.debugFailed': '無法開始除錯',
   'chat.chatInput.toast.weighFailed': '無法權衡方案',
+  'chat.chatInput.toast.exploreFailed': '無法開始導覽',
   'chat.chatInput.toast.attachmentsTooLarge': '附件過大，無法傳送。請減少圖片數量或大小。',
   'chat.chatInput.toast.sendAttachmentsFailed': '傳送附件失敗。請嘗試更少檔案或更小圖片。',
   'chat.chatInput.toast.messageSendFailed': '訊息傳送失敗，附件已恢復。',