Encode investigation source as contiguous plain-text code blocks

docs/investigation-context-v5.md

@@ -1,5 +1,8 @@
 # Investigation context v5
 
+Historical v5 format; the current measurement CLI defaults to v7. Use
+`--target-version investigation_project_v5` to reproduce this version.
+
 New project-backed investigation contexts use `investigation_project_v5`.
 Existing frozen contexts keep their version and exact request reconstruction.
 No model/provider, hypothesis budgets, money limits, lifecycle or evidence