feat: reader-rendering correctness + privacy hardening (P1·S batch)

Close the five open P1·S items from TODO.md:
- X-Robots-Tag: noindex on rss/atom/entries/files + a /robots.txt
- absolutize relative content URLs against the sender's site
- promote lazy-loaded images (data-src → src, strip loading="lazy")
- strip XML-illegal control chars from generated feeds (keep emoji)
- plain-text feed <title> (strip HTML, decode entities)

Sender-base derivation lives on the EmailAddress value object
(siteBaseUrl) instead of a misplaced favicon helper. Bump to 0.2.1
and document the changes in README + CLAUDE.md.

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

src/routes/entries.test.ts

@@ -170,4 +170,11 @@ describe("GET /entries/:feedId/:entryId", () => {
       "default-src 'none'",
     );
   });
+
+  it("sets X-Robots-Tag: noindex", async () => {
+    await seedFeed(env);
+    const app = makeApp();
+    const res = await app.request(`/${FEED_ID}/${RECEIVED_AT}`, {}, env as any);
+    expect(res.headers.get("X-Robots-Tag")).toBe("noindex");
+  });
 });