feat: split playbook into individual recipe pages (#129)

Split the monolithic playbook chapter into 11 individual recipe pages
under chapters/playbook/, each with its own URL for independent SEO
ranking. Updated cross-references in glossary and appendix, and
extended the remark link-rewriting plugin to handle subdirectory and
sibling file links.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: braboj

astro-site/src/plugins/remark-rewrite-links.ts

@@ -1,11 +1,28 @@
 import { visit } from "unist-util-visit";
 import type { Root, Link } from "mdast";
 
-const CHAPTER_LINK = /^(\d{2}-)(.+)\.md(#.*)?$/;
+const CHAPTER_LINK = /^(?:\.\.\/)?(\d{2}-)(.+)\.md(#.*)?$/;
+const SUBDIR_FILE_LINK = /^([\w-]+)\/([\w-]+)\.md(#.*)?$/;
+const SIBLING_LINK = /^([a-z][\w-]*)\.md(#.*)?$/;
 
 export function remarkRewriteLinks() {
   return (tree: Root) => {
     visit(tree, "link", (node: Link) => {
+      // Links from top-level to subdir: playbook/index.md → ../playbook/
+      // Links from top-level to subdir page: playbook/undoing-changes.md → ../playbook/undoing-changes/
+      const subdirMatch = node.url.match(SUBDIR_FILE_LINK);
+      if (subdirMatch) {
+        const slug = subdirMatch[2] === "index" ? "" : `${subdirMatch[2]}/`;
+        node.url = `../${subdirMatch[1]}/${slug}${subdirMatch[3] || ""}`;
+        return;
+      }
+      // Links to sibling files in same directory: slug.md → slug/
+      const siblingMatch = node.url.match(SIBLING_LINK);
+      if (siblingMatch) {
+        node.url = `${siblingMatch[1]}/${siblingMatch[2] || ""}`;
+        return;
+      }
+      // Links to numbered chapters: NN-slug.md or ../NN-slug.md → ../slug/
       const match = node.url.match(CHAPTER_LINK);
       if (match) {
         node.url = `../${match[2]}/${match[3] || ""}`;