feat(assets): embedded prompt and xlsx asset fallback (U10a slice 2)

- configs package embeds agents/ + shared/ prompt assets; templates package
  embeds xlsx/ (cdo_quote.xlsx)
- template.GetAgentInstructionFile: disk-first with embedded fallback for
  instructions and writing.md (dev behavior unchanged)
- quote.CalculateAndFill: embedded xlsx fallback when the on-disk template
  walk misses (single-binary deployments)
- presence test guards the embedded asset set
- verified: vet + template/quote/bizdev/squad/arch green

configs/embed.go

@@ -0,0 +1,10 @@
+// configs 包内嵌运行提示词资产（U10a：单文件应用无源码目录时回退使用）。
+// 磁盘路径存在时仍优先磁盘（开发态行为不变）。
+package configs
+
+import "embed"
+
+// Assets 内嵌 configs/agents 与 configs/shared 提示词。
+//
+//go:embed agents shared
+var Assets embed.FS