is_shell_command_allowed

Check if a shell command is in the allow-list.

The allow-list matches against the first token of the command (the executable name). This allows read-only commands like ls, cat, grep, etc. to be auto-approved.

When allow_list is the SHELL_ALLOW_ALL sentinel, all non-empty commands are approved unconditionally — dangerous pattern checks are skipped.

SECURITY: For regular allow-lists, this function rejects commands containing dangerous shell patterns (command substitution, redirects, process substitution, etc.) BEFORE parsing, to prevent injection attacks that could bypass the allow-list.