feat: user-customizable draft welcome starters

Let users curate the draft welcome chips: pin existing commands and skills
as starters, remove them, and drag to reorder — all inline on the draft
screen via a '+' picker dialog and per-chip remove, with no separate
settings UI.

A starter references a command or skill; its scope is inherited from the
item (user-scope -> global, project-scope -> per-project). Global starters
persist to settings.json (useUIStore + client/server sanitizers); project
starters persist to the project config alongside worktree setup commands.
The two scopes form ordered namespaces shown global-first then project,
reorderable only within each group.

The six built-in Session magic-prompt commands are the default global set
and stay available in the picker for re-pinning if removed; they keep their
bespoke icons, while user commands/skills fall back to the Commands/Skills
section icons. Chip labels are normalized (/simplify-code -> 'Simplify
code'). Missing commands/skills are skipped rather than shown broken.

Drag-to-reorder works on desktop and mobile: rectSortingStrategy for the
wrapping multi-row layout, CSS.Translate (no scale) so the lifted chip
doesn't stretch, and MouseSensor + long-press TouchSensor so taps still
submit and swipes still scroll. The '+' picker is a searchable dialog on
every surface.

packages/ui/src/lib/persistence.ts

@@ -8,6 +8,7 @@ import { setFilesViewShowGitignored } from '@/lib/filesViewShowGitignored';
 import { loadAppearancePreferences, applyAppearancePreferences } from '@/lib/appearancePersistence';
 import { getRegisteredRuntimeAPIs } from '@/contexts/runtimeAPIRegistry';
 import { normalizeMobileKeyboardMode, setStoredMobileKeyboardMode } from '@/lib/mobileKeyboardMode';
+import { sanitizeStarterRefs } from '@/lib/draftStarters';
 
 const persistToLocalStorage = (settings: DesktopSettings) => {
   if (typeof window === 'undefined') {
@@ -484,6 +485,12 @@ const applyDesktopUiPreferences = (settings: DesktopSettings) => {
   if (typeof settings.fontSize === 'number' && Number.isFinite(settings.fontSize) && settings.fontSize !== store.fontSize) {
     store.setFontSize(settings.fontSize);
   }
+  if (Array.isArray(settings.draftStarters)) {
+    const nextStarters = sanitizeStarterRefs(settings.draftStarters);
+    if (JSON.stringify(store.globalDraftStarters) !== JSON.stringify(nextStarters)) {
+      store.setGlobalDraftStarters(nextStarters);
+    }
+  }
   if (typeof settings.terminalFontSize === 'number' && Number.isFinite(settings.terminalFontSize) && settings.terminalFontSize !== store.terminalFontSize) {
     store.setTerminalFontSize(settings.terminalFontSize);
   }
@@ -646,6 +653,9 @@ const sanitizeWebSettings = (payload: unknown): DesktopSettings | null => {
       )
     );
   }
+  if (Array.isArray(candidate.draftStarters)) {
+    result.draftStarters = sanitizeStarterRefs(candidate.draftStarters);
+  }
   if (typeof candidate.showReasoningTraces === 'boolean') {
     result.showReasoningTraces = candidate.showReasoningTraces;
   }