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.test.ts

@@ -1,37 +1,8 @@
 import { describe, it, expect } from "vitest";
 import { EmailParser } from "./email-parser";
 
-describe("EmailParser.extractFeedId", () => {
-  it("extracts a valid feed ID from an email address", () => {
-    expect(
-      EmailParser.extractFeedId("[email protected]")?.value,
-    ).toBe("river.castle.42");
-  });
-
-  it("is case-insensitive for the local part", () => {
-    expect(
-      EmailParser.extractFeedId("[email protected]")?.value,
-    ).toBe("River.Castle.42");
-  });
-
-  it("returns null for an address with no feed ID format", () => {
-    expect(EmailParser.extractFeedId("[email protected]")).toBeNull();
-  });
-
-  it("returns null for a plain string without @", () => {
-    expect(EmailParser.extractFeedId("notanemail")).toBeNull();
-  });
-
-  it("returns null when the numeric suffix is only one digit", () => {
-    expect(EmailParser.extractFeedId("[email protected]")).toBeNull();
-  });
-
-  it("returns null when the numeric suffix has more than two digits", () => {
-    expect(
-      EmailParser.extractFeedId("[email protected]"),
-    ).toBeNull();
-  });
-});
+// Inbound mailbox parsing lives on the MailboxId VO (see mailbox-id.test.ts);
+// EmailParser no longer wraps it.
 
 describe("EmailParser.decodeEncodedWords", () => {
   it("returns plain text unchanged", () => {