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/internal/domain/design/research.go

@@ -6,6 +6,7 @@ type ResearchResult struct {
 	Title   string `json:"title"`
 	URL     string `json:"url"`
 	Snippet string `json:"snippet"`
+	Content string `json:"content,omitempty"`
 }
 
 type ResearchReport struct {