enable llms.txt generation with starlight plugin

This GEO optimization change enables llms.txt generation by integrating
`starlight-llms-txt` into the Starlight configuration.

What changed:
- added `starlight-llms-txt` dependency
- imported the plugin in `astro.config.ts`
- registered `starlightLlmsTxt()` in Starlight `plugins`

Why:
- generate `llms.txt`, `llms-full.txt`, and `llms-small.txt` directly from docs
- keep default plugin behavior first and only tune `promote`/`exclude` if needed

Manual testing:
- ran `bun lint`
- ran `bun run build`
- verified generated outputs in `dist/`: `llms.txt`, `llms-full.txt`, `llms-small.txt`

Special considerations:
- benchmark loop artifacts were intentionally not kept in the repository
- no Search Console/Bing submission changes are included here

Author: mkaput

astro.config.ts

@@ -1,5 +1,6 @@
 import { defineConfig } from "astro/config";
 import starlight from "@astrojs/starlight";
+import starlightLlmsTxt from "starlight-llms-txt";
 
 const site = "https://agentic-engineering.swmansion.com/";
 const repo = `https://github.com/software-mansion/agentic-engineering/`;
@@ -89,6 +90,7 @@ export default defineConfig({
           attrs: { name: "twitter:creator", content: "@swmansion" },
         },
       ],
+      plugins: [starlightLlmsTxt()],
     }),
   ],
 });

bun.lock

@@ -14,7 +14,8 @@
   "dependencies": {
     "@astrojs/starlight": "^0.37.6",
     "astro": "^5.6.1",
-    "sharp": "^0.34.2"
+    "sharp": "^0.34.2",
+    "starlight-llms-txt": "^0.7.0"
   },
   "devDependencies": {
     "@astrojs/check": "^0.9.6",