prompts: add heterogeneous fan-out pattern to subagents fragment

prompts/08_subagents.md

@@ -37,10 +37,22 @@ Prompt is all remaining arguments after flags.
 - N = 1: raw result
 - N > 1: results separated by `=== {job-id} ===` headers, in submission order
 
+## Heterogeneous fan-out
+
+To run N subagents with N different prompts, combine with `execute_code` parallel steps — each step is one `subagent_spawn` shell invocation:
+
+```
+execute_code: steps=[
+  {code: "subagent_spawn 'Summarize core/pkg/agent/loop.go'"},
+  {code: "subagent_spawn 'Summarize core/pkg/backend/anthropic.go'"},
+  {code: "subagent_spawn 'Summarize 9p/internal/p9/server.go'"}
+]
+```
+
+Three concurrent subagents, three different prompts. Results appear under `=== step N ===` headers from `execute_code`.
+
 ## Design rules
 
 **Same agent across parallel subagents.** Different agents produce different output schemas; concatenating them is incoherent.
 
 **Prompt for structured output when you need to parse results.** Tell subagents the exact format to respond in (numbered list, JSON, markdown sections) so you can extract what you need from the joined block.
-
-**All subagents receive the same prompt.** Differentiate their work inside the prompt — include a shard index, file path, or role description if the tasks differ.