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 <noreply@anthropic.com>

src/infrastructure/feed-generator.test.ts

@@ -10,6 +10,7 @@ const mockFeedConfig: FeedConfig = {
   title: "Test Newsletter",
   description: "A test feed",
   language: "en",
+  mailbox_id: "test.news.42",
   created_at: 1700000000000,
 };
 
@@ -146,14 +147,15 @@ describe("generateRssFeed", () => {
     expect(result).not.toContain("<item>");
   });
 
-  it("feed link points to admin emails page", () => {
+  it("feed link points to the public read URL, never an admin path", () => {
     const result = generateRssFeed(
       mockFeedConfig,
       mockEmails,
       BASE_URL,
       FEED_ID,
     );
-    expect(result).toContain(`${BASE_URL}/admin/feeds/${FEED_ID}/emails`);
+    expect(result).toContain(`<link>${BASE_URL}/rss/${FEED_ID}</link>`);
+    expect(result).not.toContain("/admin/");
   });
 
   it("strips html/head/body wrapper from item description", () => {
@@ -263,14 +265,15 @@ describe("generateAtomFeed", () => {
     expect(result).not.toContain("<entry>");
   });
 
-  it("feed link points to admin emails page", () => {
+  it("feed link points to the public read URL, never an admin path", () => {
     const result = generateAtomFeed(
       mockFeedConfig,
       mockEmails,
       BASE_URL,
       FEED_ID,
     );
-    expect(result).toContain(`${BASE_URL}/admin/feeds/${FEED_ID}/emails`);
+    expect(result).toContain(`${BASE_URL}/rss/${FEED_ID}`);
+    expect(result).not.toContain("/admin/");
   });
 
   it("strips html/head/body wrapper from entry content", () => {