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/en.settings.ts

@@ -223,6 +223,7 @@ export const settingsDict = {
   'settings.magicPrompts.sidebar.item.planImplement': 'Implement Plan',
   'settings.magicPrompts.sidebar.item.sessionSummary': 'Session Summary',
   'settings.magicPrompts.sidebar.item.sessionWorkspaceReview': 'Workspace Review',
+  'settings.magicPrompts.sidebar.item.sessionExplore': 'Codebase Tour',
   'settings.magicPrompts.sidebar.item.sessionFeaturePlan': 'Feature Planning',
   'settings.magicPrompts.sidebar.item.sessionCatchUp': 'Catch Up',
   'settings.magicPrompts.sidebar.item.sessionDebug': 'Debugging',
@@ -1689,6 +1690,8 @@ export const settingsDict = {
   'settings.magicPrompts.page.group.sessionSummary.title': 'Session Summary',
   'settings.magicPrompts.page.group.sessionSummary.description': 'Prompts used by the /summary slash command: visible user message + hidden instructions. Non-destructive - does not compact session history.',
   'settings.magicPrompts.page.group.sessionWorkspaceReview.title': 'Workspace Review',
+  'settings.magicPrompts.page.group.sessionExplore.title': 'Codebase Tour',
+  'settings.magicPrompts.page.group.sessionExplore.description': 'Prompts used by the /explore slash command: visible user message + hidden instructions. Investigates the repository and gives a structured orientation — the big picture, main modules, how they connect, and where to start — rather than a file-by-file dump.',
   'settings.magicPrompts.page.group.sessionWorkspaceReview.description': 'Prompts used by the /workspace-review slash command: visible user message + hidden instructions. Reviews current workspace changes for high-signal issues only.',
   'settings.magicPrompts.page.group.sessionFeaturePlan.title': 'Feature Planning',
   'settings.magicPrompts.page.group.sessionFeaturePlan.description': 'Prompts used by the /plan-feature slash command: visible user message + hidden instructions. Runs a guided dialogue that researches the code and asks clarifying questions in small batches before producing an implementation plan.',