Improve sitemap with per-URL lastmod and include guides

[anurag629]

Context

Part of the SEO growth initiative (#18): Phase 4: Compound and technical polish.
A single build-time lastModified for every URL is a weak freshness signal. Per-URL dates and including guides make the sitemap more useful.

What to do

• Derive lastModified per URL from content/git rather than one build timestamp.
• Include guide URLs once the guides system exists.

Where

app/sitemap.ts; coordinate with the guides registry.

Acceptance criteria

• Sitemap emits per-URL lastmod and includes guides
• npm run lint && npm run test && npm run build passes

Notes

Branch off develop (feat/*, docs/*, chore/*), open a PR into develop, and squash-merge. See CONTRIBUTING.md.