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/value-objects/mailbox-id.test.ts

@@ -0,0 +1,50 @@
+import { describe, it, expect } from "vitest";
+import { MailboxId } from "./mailbox-id";
+
+describe("MailboxId.parse", () => {
+  it("extracts the mailbox id from an inbound address", () => {
+    expect(MailboxId.parse("[email protected]")?.value).toBe(
+      "river.castle.42",
+    );
+  });
+
+  it("preserves the original casing of the local part", () => {
+    expect(MailboxId.parse("[email protected]")?.value).toBe(
+      "River.Castle.42",
+    );
+  });
+
+  it("rejects malformed mailbox ids", () => {
+    expect(MailboxId.parse("[email protected]")).toBeNull();
+    expect(MailboxId.parse("notanemail")).toBeNull();
+    expect(MailboxId.parse("[email protected]")).toBeNull();
+    expect(MailboxId.parse("[email protected]")).toBeNull();
+  });
+});
+
+describe("MailboxId.generate", () => {
+  it("produces the noun.noun.NN format", () => {
+    for (let i = 0; i < 50; i++) {
+      expect(MailboxId.generate().value).toMatch(/^[a-z]+\.[a-z]+\.\d{2}$/);
+    }
+  });
+
+  it("round-trips through parse from an address", () => {
+    const id = MailboxId.generate();
+    expect(MailboxId.parse(`${id.value}@example.com`)?.value).toBe(id.value);
+  });
+});
+
+describe("MailboxId.unchecked", () => {
+  it("wraps a value without validation", () => {
+    expect(MailboxId.unchecked("anything").value).toBe("anything");
+  });
+});
+
+describe("MailboxId.emailAddress", () => {
+  it("builds the full inbound address from the mailbox and a domain", () => {
+    expect(
+      MailboxId.unchecked("river.castle.42").emailAddress("news.app"),
+    ).toBe("[email protected]");
+  });
+});