Hardened subprocess stdio wiring and failure diagnostics

architecture.md

@@ -333,9 +333,9 @@ All adapters currently have fake/no-op implementations for tests/scaffold execut
 - Context cancellation + optional timeout.
 - Explicit executable/args, working dir, env overrides.
 - Parent environment inheritance with override merge semantics (override values win).
-- Stdout/stderr log file handling.
+- Stdout/stderr log file handling (including a shared-stream guard when both logs target the same path).
 - Exit code and timing capture.
-- Actionable error wrapping.
+- Actionable error wrapping, including stdout/stderr log paths and a short redacted stderr tail when available.
 - Atomic file/YAML writers for generated config/log scaffolding.
 
 ## 12. Run Control, Locking, Logging, and Long-Running Stages