feat(nodes): add Bash node for running shell commands

[xujustinj]

Closes #157.

Implements the idea from #157: every shell command can be viewed as a workflow node with arguments in and its standard streams as output files.

What's new

BashNode (type: "Bash"):

• command param — the shell command, rendered as a Jinja template so values from upstream nodes substitute in via {{ name }} (StrictUndefined, so a missing variable errors instead of blanking).
• combine_output param — when true, merges stderr into stdout (2>&1) and switches the node's output shape to a single combined output file (via dynamic_output_type), as described in the issue.
• arguments input — a StringMapValue[StringValue] (defaults empty, so argument-free commands need no edge) supplying the Jinja variables.
• Outputs — stdout + stderr files and an exit_code, or a single output file + exit_code when combining.
• Runs in the user's $SHELL (falls back to /bin/bash) in the engine's own environment, matching the maintainer discussion on the issue.

Security

This is effectively remote code execution — it runs arbitrary commands in the engine's environment. The node/module descriptions and docs flag it as trusted, locally-operated engines only; never expose it on an engine that runs untrusted workflows or accepts workflows over the network.

Notes / open questions

• Non-zero exits don't raise — exit_code is just an output, leaving failure handling to downstream nodes. Happy to add a check param that raises on non-zero if preferred.
• .bashrc sourcing — the issue mused about full interactive-shell init. This uses a plain non-interactive bash -c (no -i/-l) to avoid hangs and job-control noise; users add init via && as the issue suggests.

Testing

New tests/test_shell.py (4 tests): basic command, Jinja arg substitution through an input edge, non-zero exit + stderr capture, combined output. Full suite green: ruff (check + format), pyright (0 errors), 648 passed.

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