Potential fix for code scanning alert no. 2: Uncontrolled command line

[Rootless-Ghost]

Potential fix for https://github.com/Rootless-Ghost/AtomicLoop/security/code-scanning/2

To fix this without changing intended functionality, keep the allowlist gate but remove avoidable shell interpretation in _build_command for bash, sh, and non-Windows cmd fallback. Instead of ["bash", "-c", command] / ["sh", "-c", command], parse command into argv with shlex.split(command) and execute directly. This prevents shell metacharacter chaining from being interpreted as new commands.

Best concrete change:

• File: core/executor.py
• Region: _build_command(...) (lines around 294–305 in snippet)
• Replace shell -c launches for:
  • non-Windows cmd fallback
  • bash
  • sh
• Use shlex.split(command, posix=(platform.system().lower() != "windows")).
• If parsing fails (ValueError) or command is empty, return None so execute() responds with unsupported/invalid command error path.
• Keep PowerShell/Windows cmd behavior unchanged to minimize functional impact for platform-native executors.

No changes are required in app.py for this specific sink hardening.

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