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>

src/reply.ts

@@ -6,7 +6,6 @@ const REPLY_DEFAULTS_KEY = "velum.reply.defaults.v1";
 export type ReplyDefaults = {
   greeting: string;
   signOff: string;
-  model: string;
   creativity: string;
   disclosure: boolean;
 };