feat(piagent): give image generator inline reference placement context

Derive an ordered map of each inline reference image to the surrounding
prompt text describing its role, and feed it into the decision, research,
task-plan, conversation and image-prompt builders. The image prompt now
carries this placement so reference roles are not swapped.

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

server/internal/infrastructure/piagent/creative_agent.go

@@ -577,6 +577,9 @@ func buildDecisionPrompt(req design.AgentDecisionRequest) string {
 		"",
 		"Input summary:",
 		agentMessageContentSummary(req.Messages),
+		"",
+		"Inline reference placement:",
+		inlineReferencePlacementContext(req.Prompt, req.Messages),
 	}, "\n")
 }
 
@@ -598,6 +601,9 @@ func buildResearchDecisionPrompt(req design.AgentResearchDecisionRequest) string
 		req.Prompt,
 		agentMessageContentSummary(req.Messages),
 		"",
+		"Inline reference placement:",
+		inlineReferencePlacementContext(req.Prompt, req.Messages),
+		"",
 		"Mode: " + req.Mode,
 	}, "\n")
 }
@@ -624,6 +630,9 @@ func buildAgentTaskPlanPrompt(req design.AgentTaskPlanRequest) string {
 		req.Prompt,
 		agentMessageContentSummary(req.Messages),
 		"",
+		"Inline reference placement:",
+		inlineReferencePlacementContext(req.Prompt, req.Messages),
+		"",
 		"Mode: " + req.Mode,
 		"Web search: " + searchState,
 		"",
@@ -648,6 +657,9 @@ func buildConversationPrompt(req design.AgentConversationRequest) string {
 		"Inline references:",
 		agentMessageContentSummary(req.Messages),
 		"",
+		"Inline reference placement:",
+		inlineReferencePlacementContext(req.Prompt, req.Messages),
+		"",
 		"User message:",
 		req.Prompt,
 		"",
@@ -697,6 +709,9 @@ func buildImagePromptPlannerInput(req design.AgentImagePromptRequest) string {
 		req.Prompt,
 		agentMessageContentSummary(req.Messages),
 		"",
+		"Inline reference placement:",
+		inlineReferencePlacementContext(req.Prompt, req.Messages),
+		"",
 		"Mode: " + req.Mode,
 	}, "\n")
 }