Improve Lighthouse Agentic Browsing readiness (#8)

## Summary

- Add `robots.txt`, `sitemap.xml`, and `llms.txt` endpoints so crawlers and AI agents can discover public pages
- Fix accessibility tree issues: contact form labels, semantic nav/profile dropdown buttons, cookie consent dialog ARIA, and dead `href="#"` links
- Reduce layout shift on homepage/contact via hero min-heights, trusted-by marquee sizing, and fixed cookie banner
- Add regression tests and `docs/agentic-browsing.md` for Lighthouse agentic-browsing audits

Closes #5. Also addresses overlap with #3 (sitemap + robots.txt).

## Test plan

- [x] `python manage.py test public.tests` (14 tests pass)
- [ ] Verify `GET /robots.txt`, `/sitemap.xml`, `/llms.txt` return 200 in staging/prod
- [ ] Confirm contact form labels visible and submittable on `/contact`
- [ ] Run Lighthouse `--only-categories=agentic-browsing` on homepage and contact page
- [ ] Smoke test mobile nav dropdown and cookie consent banner

Reviewed-on: #8

company_site/public/seo.py

@@ -0,0 +1,66 @@
+"""Machine-readable site discovery endpoints for crawlers and AI agents."""
+
+from django.http import HttpResponse
+from django.template.loader import render_to_string
+from django.urls import reverse
+
+# Public marketing pages included in sitemap and llms.txt.
+PUBLIC_PAGE_ENTRIES = (
+    ("public_index", "Home", "weekly", "1.0"),
+    ("forward_deployed", "Forward-Deployed AI", "monthly", "0.9"),
+    ("bot", "AI Agents", "monthly", "0.9"),
+    ("ml_model", "ML Models", "monthly", "0.8"),
+    ("chat", "Secure AI Chat", "monthly", "0.8"),
+    ("ai_sensor", "AI Sensor Algorithms", "monthly", "0.7"),
+    ("ai_education", "AI Education", "monthly", "0.7"),
+    ("computers", "Computer Builds", "monthly", "0.7"),
+    ("file_hosting", "File Hosting", "monthly", "0.7"),
+    ("web_design", "Web Design and Hosting", "monthly", "0.8"),
+    ("contact", "Contact", "monthly", "0.9"),
+    ("terms_of_service", "Terms of Service and Privacy", "yearly", "0.3"),
+)
+
+
+def _absolute_url(request, url_name):
+    return request.build_absolute_uri(reverse(url_name))
+
+
+def robots_txt(request):
+    sitemap_url = _absolute_url(request, "sitemap_xml")
+    content = render_to_string(
+        "public/robots.txt",
+        {"sitemap_url": sitemap_url},
+    )
+    return HttpResponse(content, content_type="text/plain; charset=utf-8")
+
+
+def sitemap_xml(request):
+    pages = [
+        {
+            "loc": _absolute_url(request, url_name),
+            "changefreq": changefreq,
+            "priority": priority,
+        }
+        for url_name, _title, changefreq, priority in PUBLIC_PAGE_ENTRIES
+    ]
+    content = render_to_string("public/sitemap.xml", {"pages": pages})
+    return HttpResponse(content, content_type="application/xml; charset=utf-8")
+
+
+def llms_txt(request):
+    pages = [
+        {
+            "title": title,
+            "url": _absolute_url(request, url_name),
+        }
+        for url_name, title, _changefreq, _priority in PUBLIC_PAGE_ENTRIES
+    ]
+    content = render_to_string(
+        "public/llms.txt",
+        {
+            "site_url": request.build_absolute_uri("/"),
+            "contact_url": _absolute_url(request, "contact"),
+            "pages": pages,
+        },
+    )
+    return HttpResponse(content, content_type="text/plain; charset=utf-8")