feat: add Atom feed generation

- src/feed.rs: New module with generate_atom_feed() that produces
  Atom 1.0 XML from blog posts. Uses config.base_url for absolute
  entry URLs. Includes xml_escape() helper.

- src/main.rs: Wire mod feed and call generate_feed() after blog
  index generation. Outputs to public/feed.xml.

- src/templates.rs: Add <link rel="alternate" type="application/atom+xml">
  autodiscovery link to page head using config.base_url.

Feed includes title, author, updated timestamp, and entries with
title, link, id, updated, and summary for each blog post.

src/main.rs

@@ -6,6 +6,7 @@ mod config;
 mod content;
 mod css;
 mod error;
+mod feed;
 mod highlight;
 mod render;
 mod templates;
@@ -45,6 +46,9 @@ fn run() -> Result<()> {
     posts.sort_by(|a, b| b.frontmatter.date.cmp(&a.frontmatter.date));
     generate_blog_index(output_dir, &posts, &config)?;
 
+    // 2b. Generate Atom feed
+    generate_feed(output_dir, &posts, &config)?;
+
     // 3. Process standalone pages (about, collab)
     process_pages(content_dir, output_dir, &config)?;
 
@@ -122,6 +126,22 @@ fn generate_blog_index(
     Ok(())
 }
 
+/// Generate the Atom feed
+fn generate_feed(output_dir: &Path, posts: &[Content], config: &config::SiteConfig) -> Result<()> {
+    let out_path = output_dir.join("feed.xml");
+    eprintln!("generating: {}", out_path.display());
+
+    let feed_xml = feed::generate_atom_feed(posts, config);
+
+    fs::write(&out_path, feed_xml).map_err(|e| Error::WriteFile {
+        path: out_path.clone(),
+        source: e,
+    })?;
+
+    eprintln!("  → {}", out_path.display());
+    Ok(())
+}
+
 /// Process standalone pages in content/ (about.md, collab.md)
 fn process_pages(content_dir: &Path, output_dir: &Path, config: &config::SiteConfig) -> Result<()> {
     for name in ["about.md", "collab.md"] {