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 <[email protected]>

src/routes/atom.ts

@@ -40,6 +40,7 @@ export async function handle(c: Context<{ Bindings: Env }>): Promise<Response> {
       headers: {
         "Content-Type": "application/atom+xml",
         "Cache-Control": "max-age=1800",
+        "X-Robots-Tag": "noindex",
         Link: linkHeader,
       },
     });