Add robots and fix up sitemap.

robots.txt

@@ -1,4 +1,11 @@
+---
+layout: null
+sitemap:
+  exclude: true
+---
+
 User-agent: *
+Sitemap: {{ "/robots.txt" | absolute_url }}
 Disallow: /assets/
 Disallow: /feed/
 Disallow: /feed.xml
@@ -13,12 +20,14 @@ User-agent: Applebot
 User-agent: Applebot-Extended
 User-agent: Bytespider
 User-agent: CCBot
+User-agent: ChatGPT
 User-agent: ChatGPT-User
 User-agent: Claude-Web
 User-agent: ClaudeBot
 User-agent: Diffbot
 User-agent: FacebookBot
 User-agent: FriendlyCrawler
+User-agent: GPT
 User-agent: GPTBot
 User-agent: Google-Extended
 User-agent: GoogleOther
@@ -29,7 +38,9 @@ User-agent: ICC-Crawler
 User-agent: ImagesiftBot
 User-agent: Meta-ExternalAgent
 User-agent: Meta-ExternalFetcher
+User-agent: OpenAI
 User-agent: OAI-SearchBot
+User-agent: OAI-Bot
 User-agent: PerplexityBot
 User-agent: PetalBot
 User-agent: Scrapy