refactor(feed): drop unused aggregate getter, dedupe sender parse

The sender-in-title rendering reads the FeedConfig DTO via the read
model, so the Feed.senderInTitle getter had no consumer — remove it.
In buildFeed, parse the from-address once and reuse it for both the
title prefix and the author; drop the dead `?? email.from` fallback
(parseFromAddress already returns the address as the name).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/domain/feed.aggregate.ts

@@ -138,11 +138,6 @@ export class Feed {
     return this._state.language;
   }
 
-  /** Whether entry titles render as `[Sender] Subject` in the feed output. */
-  get senderInTitle(): boolean {
-    return this._state.senderInTitle ?? false;
-  }
-
   /** The inbound mailbox (`noun.noun.NN`) — the feed's email address is `mailboxId@domain`. */
   get mailboxId(): MailboxId {
     return MailboxId.unchecked(this._state.mailboxId);