feat(analytics): retire Rybbit and add SEO/GEO discovery tracking

Document Umami as the sole analytics platform, remove Rybbit proxy and
tracker code, and classify public page traffic with discovery_channel for
adm SEO/GEO dashboards.

Co-authored-by: Cursor <cursoragent@cursor.com>

release-gate/coverage.mjs

@@ -942,7 +942,7 @@ export const AUTOMATION_SUITES = Object.freeze([
       'services/orchestrator/shadow-observer.test.mjs',
       'services/orchestrator/observability.test.mjs',
       'mindspace-analytics.test.mjs',
-      'mindspace-rybbit.test.mjs',
+      'mindspace-analytics-discovery.test.mjs',
       'system-disclosure-policy.test.mjs',
     ],
   },