- execute_code now takes steps:[{code,language}|{tool,args}] instead of
a single code/language pair. Single-step calls return raw output;
multi-step calls run in parallel and return results under === step N ===
headers in submission order.
- execute_pipe stages may now include {parallel:[...]} to fan out
concurrently within a pipeline. Parallel outputs are concatenated in
submission order and fed as stdin to the next stage.
- Stage execution is now Go-level (stdin chaining) rather than a single
bash pipeline string, enabling parallel stages and tool/code mixing.
- CodeStep and resolveCodeStep are the shared primitives for both
execute_code and execute_pipe parallel stages.
- executeWithStdin added to Server for stdin chaining between stages.
- Tool steps use shebang detection; language field applies only to
inline code steps (default: bash).
- ListTools schemas and descriptions updated throughout.
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