feat: persist last-used model in LocalStorage instead of preference

Drop the summaryModel preference (and the awkward textfield holding a
raw model ID nobody could discover without poking the SDK types). Each
AI view now controls its model dropdown, loads the shared
velum.ai.last-model on mount, and writes it back on submit — so picking
Claude 4.7 Opus in the summarize command becomes the prefilled default
in briefing, action-items, structured-data, and reply next time around.

Also drops 'model' from ReplyDefaults; the shared key supersedes it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

package.json

@@ -178,15 +178,6 @@
       "required": true,
       "placeholder": "Raphael"
     },
-    {
-      "name": "summaryModel",
-      "title": "Standard-Modell für AI-Befehle",
-      "description": "Raycast-KI-Modell-ID, vorausgewählt in den AI-Views. Die volle Modellliste ist dort als Dropdown verfügbar. Benötigt Raycast Pro.",
-      "type": "textfield",
-      "required": true,
-      "default": "anthropic-claude-sonnet-4-6",
-      "placeholder": "anthropic-claude-sonnet-4-6"
-    },
     {
       "name": "maxSessions",
       "title": "Maximale Anzahl gespeicherter Sitzungen",