feat: built-in bash skill (bash -c, shell parity with a native tool)

[samsja]

Summary

Adds a built-in bash skill (enabled via RLM_SKILLS): await bash(command=...) runs a shell command from the REPL and returns combined stdout/stderr as a string (exit code appended when nonzero, output clipped at 30k chars).

Why: lets the agent mix shell with Python in one cell — capture output as a value, parse it programmatically, or sidestep shell-quoting by building commands as triple-quoted Python strings.

Implementation note: executes via ["bash", "-c", command] explicitly rather than create_subprocess_shell, which resolves to /bin/sh (dash on debian images) and rejects bashisms — we hit sh: Syntax error: "(" unexpected on process substitution in ablation runs before pinning this down. A parity test (cat <(echo ...)) guards it.

Tests: 4 new (output capture, bashism parity, exit-code reporting, stderr merging); suite green except 6 pre-existing test_acp.py failures that also fail on clean main in this environment.

Extracted from #142 so it can ship independently.

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Note

Medium Risk
Introduces arbitrary shell execution from the REPL; mitigated by shared git-history blocking and timeouts, but still expands the agent’s command surface.

Overview
Adds a bash built-in skill (enable with RLM_SKILLS) so agents can await bash(...) from IPython and get shell output as a string instead of only using separate shell tools.

The new rlm.skills.bash module runs one command per call via bash -c (not /bin/sh), merges stdout and stderr, appends [exit code N] on failure, honors RLM_EXEC_TIMEOUT (default 300s), and applies the same find_blocked_command / git-history guard as other shell paths. Registration in _BUILTIN_SKILLS wires it into the existing session stub / pre-import flow like edit.

Tests cover echo output, bash-specific syntax (process substitution), nonzero exits, stderr, git guard behavior, timeouts, and that enable_builtin_skills(["bash"], ...) writes the expected re-export stub.

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[samsja]

Fixed in bab8cf6 — the registration edit had targeted the pre-rename dict shape and silently never applied. bash is now in _BUILTIN_SKILLS and the registration path is covered by a stub-writing test (test_enable_bash_skill_writes_stub). Good catch.