feat(piagent): batch-generate images in one call when generators share a prompt

When multiple image-generator tasks carry the same brief, request all
images from a single GPT Image call with Count=N instead of firing one
call per image. Adds directImageBatchCount/Prompt detection, batch
(incremental) generation paths, and folds the planned count into the
idempotency key. Disables the streaming plan probe for multi-image
requests.

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

server/internal/infrastructure/piagent/image_generation.go

@@ -136,7 +136,7 @@ func (c *ImageClient) Generate(ctx context.Context, prompt string, opts ImageReq
 
 	idempotencyKey := strings.TrimSpace(opts.IdempotencyKey)
 	streamPlans := []bool{false}
-	if len(images) == 0 {
+	if len(images) == 0 && count == 1 {
 		streamPlans = []bool{true, false}
 	}