feat(research): hydrate web-search results with cleaned page content

Fetch and clean each search result's page body (bounded by size,
concurrency, and a configurable MaxPageContentRunes), store it on the
new ResearchResult.Content field, and surface those excerpts in agent
memory so the model reasons over real page text rather than snippets
alone. Promotes golang.org/x/net to a direct dependency for HTML
parsing.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/go.mod

@@ -16,6 +16,7 @@ require (
 	github.com/zeromicro/go-zero v1.10.2
 	golang.org/x/crypto v0.51.0
 	golang.org/x/image v0.41.0
+	golang.org/x/net v0.53.0
 )
 
 require (
@@ -70,7 +71,6 @@ require (
 	go.uber.org/automaxprocs v1.6.0 // indirect
 	go.yaml.in/yaml/v2 v2.4.2 // indirect
 	go.yaml.in/yaml/v3 v3.0.4 // indirect
-	golang.org/x/net v0.53.0 // indirect
 	golang.org/x/sync v0.20.0 // indirect
 	golang.org/x/sys v0.44.0 // indirect
 	golang.org/x/text v0.37.0 // indirect