Remove unused alert envelope retention

docs/internal/facts.md

@@ -16,6 +16,9 @@ story, and convective outlook data. Source provenance and warnings are copied
 into their own slices so downstream consumers can inspect data completeness
 without treating it as an ordinary weather fact.
 
+Alert facts retain individual alert payloads for period selection together with
+their copied source provenance; they do not retain a provider response envelope.
+
 A nil bundle produces an empty collected value. Collection itself, missing
 source policy, and source hashes are outside this package.