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/index.ts

@@ -184,6 +184,14 @@ app.get("/health", (c) => c.json({ status: "ok", timestamp: Date.now() }));
 // Public status page (counters + link to admin)
 app.get("/", handleHome);
 
+// Keep private feeds/emails out of search engines (defense in depth alongside
+// the X-Robots-Tag headers on the feed/entry/file responses).
+app.get("/robots.txt", (c) =>
+  c.text(
+    "User-agent: *\nDisallow: /rss/\nDisallow: /atom/\nDisallow: /entries/\nDisallow: /files/\nDisallow: /admin/\n",
+  ),
+);
+
 // Catch-all for 404s
 app.all("*", (c) => c.text("Not Found", 404));