feat(agent): clarify vague briefs before generating and harden prompts

Let the planner return a clarify/chat intent that asks 2-4 targeted
questions (imageCount 0) when essentials are missing, instead of forging
ahead. Tighten prompt guidance to preserve user wording and stated
reference-image roles, avoid inventing copy/specs, keep durable brand
constraints, and treat vague taste feedback as clarification. Also
fix the generation heartbeat goroutine to stop cleanly via defer, add
research-query fallbacks, and refresh image-only capability copy.

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

server/internal/infrastructure/piagent/creative_agent_test.go

@@ -1,6 +1,7 @@
 package piagent
 
 import (
+	"encoding/json"
 	"strings"
 	"testing"
 	"unicode/utf8"
@@ -37,3 +38,33 @@ func TestAgentMessageContentSummaryDoesNotLeakImageURLs(t *testing.T) {
 		t.Fatalf("expected compact inline text summary, got %d runes", utf8.RuneCountInString(summary))
 	}
 }
+
+func TestResearchContextIncludesPageContentForEveryResult(t *testing.T) {
+	payload, err := json.Marshal(design.ResearchReport{
+		Kind:   "web_search",
+		Source: "test",
+		Query:  "Japanese bakery brand identity",
+		Results: []design.ResearchResult{
+			{Title: "Bakery case study", URL: "https://example.com/one", Snippet: "Warm handmade identity.", Content: "Detailed first webpage content about packaging, signage, and illustration."},
+			{Title: "Brand identity notes", URL: "https://example.com/two", Snippet: "Soft visual system.", Content: "Detailed second webpage content about colors, typography, and bakery tone."},
+		},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	context := researchContext([]design.Message{
+		{Role: "tool", Type: "tool", Title: "Synthesize Skills", Content: string(payload)},
+	})
+	for _, want := range []string{
+		"Query: Japanese bakery brand identity",
+		"https://example.com/one",
+		"Detailed first webpage content",
+		"https://example.com/two",
+		"Detailed second webpage content",
+	} {
+		if !strings.Contains(context, want) {
+			t.Fatalf("expected research context to include %q, got %s", want, context)
+		}
+	}
+}