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/application/agent_memory.go

@@ -238,6 +238,7 @@ func researchMemory(messages []design.Message) string {
 				Title   string `json:"title"`
 				URL     string `json:"url"`
 				Snippet string `json:"snippet"`
+				Content string `json:"content"`
 			} `json:"results"`
 		}
 		if err := json.Unmarshal([]byte(message.Content), &report); err != nil {
@@ -255,6 +256,9 @@ func researchMemory(messages []design.Message) string {
 			if snippet := strings.TrimSpace(result.Snippet); snippet != "" {
 				line += " | " + shortenMemoryText(snippet, 160)
 			}
+			if content := strings.TrimSpace(result.Content); content != "" {
+				line += " | page: " + shortenMemoryText(content, 360)
+			}
 			lines = append(lines, line)
 			if len(lines) >= 10 {
 				break