feat: decouple read FeedId from inbound MailboxId

Separate the two feed identities so the public read URL never reveals the
inbound address and vice-versa:

- FeedId becomes an opaque high-entropy token (read id + KV key); MailboxId
  (noun.noun.NN) owns the inbound address and the untrusted-input boundary
  via MailboxId.parse. They map only through the inbound:<mailbox> secondary
  index, resolved solely at reception.
- inbound index lifecycle is owned by FeedRepository: written by save/saveConfig,
  dropped by removeFromList(Bulk) — symmetric, never mirrored by hand (removes the
  manual delete in feed-service + the cron loop, and a silent empty-catch).
- Feed.mailboxId exposes a MailboxId VO (symmetry with Feed.id); the
  mailbox@domain shape lives on MailboxId.emailAddress(domain).
- Distinguish mailbox_unknown (no feed claims the address) from feed_not_found
  (dangling index) for observability; both forwardable, both 404.
- Drop the redundant EmailParser.extractMailbox pass-through so MailboxId.parse
  is the single parse boundary.

Docs (README/INSTALL/CLAUDE.md/landing) and tests updated; 439 tests green,
tsc clean, build dry-run OK.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

src/domain/email-parser.ts

@@ -1,17 +1,6 @@
 import { EmailData } from "../types";
-import { FeedId } from "./value-objects/feed-id";
 
 export class EmailParser {
-  /**
-   * Extract the feed id from an inbound recipient address. Returns a validated
-   * `FeedId` value object (not a raw string) so the most untrusted input in the
-   * system — an address typed by a sender — is guarded at the parse boundary and
-   * never needs `FeedId.unchecked` downstream.
-   */
-  static extractFeedId(emailAddress: string): FeedId | null {
-    return FeedId.parse(emailAddress);
-  }
-
   // eslint-disable-next-line @typescript-eslint/no-explicit-any
   static parseForwardEmailPayload(payload: any): EmailData {
     if (!payload) {