feat: integrate tag pages into sitemap

Thread tag names from collect_tags() through
generate_sitemap_file to generate_sitemap, appending
/tags/<tag>.html entries to sitemap.xml output.

Refactor: hoist collect_tags into run(), rename
generate_tag_pages to write_tag_pages.

Test suite: 83 → 84, phase 3 complete.

docs/plans/api-stabilization.md

@@ -131,10 +131,10 @@ Items validated by codebase investigation:
    - [x] Collect all unique tags across content items during build
    - [x] Create `tags/default.html` template in `docs/templates/`
    - [x] Generate `/tags/<tag>.html` for each unique tag with list of tagged items
-   - [ ] Add tag listing page entries to sitemap (if enabled)
+   - [x] Add tag listing page entries to sitemap (if enabled)
    - [x] Add tests: 404 page generation
    - [x] Add tests: tag listing page generation (correct paths, correct items per tag)
-   - [ ] End-to-end: build `docs/` site and verify all outputs
+   - [x] End-to-end: build `docs/` site and verify all outputs
 
 ## Verification
 

src/main.rs

@@ -200,12 +200,16 @@ fn run(config_path: &Path) -> Result<()> {
         generate_404(page_404, &output_dir, &config, &manifest.nav, &engine)?;
     }
 
-    // 6. Generate tag listing pages
-    generate_tag_pages(&output_dir, &content_dir, &config, &manifest, &engine)?;
+    // 6. Collect tags and generate tag listing pages
+    let tags = collect_tags(&manifest.sections, &manifest.pages, &content_dir, &config);
+    if !tags.is_empty() {
+        write_tag_pages(&output_dir, &tags, &config, &manifest.nav, &engine)?;
+    }
 
     // 7. Generate sitemap (if enabled)
+    let tag_names: Vec<String> = tags.keys().cloned().collect();
     if config.sitemap.enabled {
-        generate_sitemap_file(&output_dir, &manifest, &config, &content_dir)?;
+        generate_sitemap_file(&output_dir, &manifest, &config, &content_dir, &tag_names)?;
     }
 
     // 8. Generate alias redirects
@@ -242,11 +246,12 @@ fn generate_sitemap_file(
     manifest: &content::SiteManifest,
     config: &config::SiteConfig,
     content_dir: &Path,
+    tag_names: &[String],
 ) -> Result<()> {
     let out_path = output_dir.join("sitemap.xml");
     eprintln!("generating: {}", out_path.display());
 
-    let sitemap_xml = sitemap::generate_sitemap(manifest, config, content_dir);
+    let sitemap_xml = sitemap::generate_sitemap(manifest, config, content_dir, tag_names);
 
     fs::write(&out_path, sitemap_xml).map_err(|e| Error::WriteFile {
         path: out_path.clone(),
@@ -387,29 +392,23 @@ fn collect_tags(
     tags
 }
 
-/// Generate tag listing pages for all unique tags across content.
-fn generate_tag_pages(
+/// Write tag listing pages from pre-collected tag data.
+fn write_tag_pages(
     output_dir: &Path,
-    content_dir: &Path,
+    tags: &BTreeMap<String, Vec<ContentContext>>,
     config: &config::SiteConfig,
-    manifest: &content::SiteManifest,
+    nav: &[NavItem],
     engine: &TemplateEngine,
 ) -> Result<()> {
-    let tags = collect_tags(&manifest.sections, &manifest.pages, content_dir, config);
-
-    if tags.is_empty() {
-        return Ok(());
-    }
-
     let tags_dir = output_dir.join("tags");
     fs::create_dir_all(&tags_dir).map_err(|e| Error::CreateDir {
         path: tags_dir.clone(),
         source: e,
     })?;
 
-    for (tag, items) in &tags {
+    for (tag, items) in tags {
         let page_path = format!("/tags/{}.html", tag);
-        let html = engine.render_tag_page(tag, items, &page_path, config, &manifest.nav)?;
+        let html = engine.render_tag_page(tag, items, &page_path, config, nav)?;
 
         let out_path = tags_dir.join(format!("{}.html", tag));
         fs::write(&out_path, html).map_err(|e| Error::WriteFile {

src/sitemap.rs

@@ -24,6 +24,7 @@ pub fn generate_sitemap(
     manifest: &SiteManifest,
     config: &SiteConfig,
     content_root: &Path,
+    tag_names: &[String],
 ) -> String {
     let base_url = config.base_url.trim_end_matches('/');
     let mut entries = Vec::new();
@@ -63,6 +64,14 @@ pub fn generate_sitemap(
         });
     }
 
+    // Tag listing pages
+    for tag in tag_names {
+        entries.push(SitemapEntry {
+            loc: format!("{}/tags/{}.html", base_url, tag),
+            lastmod: None,
+        });
+    }
+
     build_sitemap_xml(&entries)
 }
 
@@ -165,4 +174,22 @@ mod tests {
         assert!(xml.contains("&"));
         assert!(!xml.contains("?q=foo&bar")); // Raw & should not appear
     }
+
+    #[test]
+    fn test_sitemap_includes_tag_pages() {
+        let tag_names = vec!["rust".to_string(), "web".to_string()];
+        let entries: Vec<SitemapEntry> = tag_names
+            .iter()
+            .map(|tag| SitemapEntry {
+                loc: format!("https://example.com/tags/{}.html", tag),
+                lastmod: None,
+            })
+            .collect();
+
+        let xml = build_sitemap_xml(&entries);
+
+        assert!(xml.contains("https://example.com/tags/rust.html"));
+        assert!(xml.contains("https://example.com/tags/web.html"));
+        assert_eq!(xml.matches("<url>").count(), 2);
+    }
 }