[plan] GEO: Add sitemap.xml, generate sitemap, and improve README brand signals

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Objective

Fix the missing sitemap (404 on sitemap.xml), and improve README brand & entity signals including RSS feed link and freshness date.

Context

From the GEO Audit Report in discussion #32848:

1. Docs site sitemap — https://github.github.com/gh-aw/sitemap.xml returns 404. The robots.txt references sitemap-index.xml but the audit tool expects sitemap.xml. The Astro config imports @astrojs/sitemap so a sitemap should be generated.
2. README signals — Score 3/6. Missing: RSS feed link, freshness date. These are achievable improvements.
3. README brand signals — Score 3/10. The audit notes "About link present, no KG sameAs".

Approach

Fix Sitemap

1. Check what sitemap files are actually generated by the Astro build:

   cd docs && npm run build && ls dist/sitemap*.xml

2. The @astrojs/sitemap plugin with a base path may generate sitemap-index.xml + sitemap-0.xml. Ensure sitemap.xml is also created (either as the main file or as a redirect/alias).
3. Update docs/public/robots.txt to reference the correct sitemap URL that actually resolves.

Improve README Signals

1. Add a freshness date to the README — add a "Last updated" date or a "Version" badge near the top
2. Add an RSS feed link — add a badge or link to the docs site's RSS feed (https://github.github.com/gh-aw/blog/rss.xml or similar) in the README
3. Add sameAs links (for brand signals) — add a "Find us at" section or links to social/brand profiles in the README

README Meta Improvements

1. The README scores 11/14 on meta tags — investigate what's missing (likely canonical URL, which is a GitHub Pages platform constraint)
2. Document the canonical URL limitation as a known platform constraint in a comment

Files to Modify

• docs/public/robots.txt — update sitemap URL to match actual generated file
• docs/astro.config.mjs — verify sitemap plugin configuration
• README.md — add freshness date, RSS link, brand sameAs signals

Acceptance Criteria

• https://github.github.com/gh-aw/sitemap.xml returns 200 with valid XML sitemap content
• robots.txt sitemap URL matches the actual accessible sitemap
• README has a freshness/last-updated indicator
• README has a link to the docs RSS feed
• README signals score improves from 3/6 toward 6/6

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• expires on May 19, 2026, 4:55 PM UTC