CVE-2026-32032: OpenClaw's shell env fallback trusts unvalidated SHELL path from host environment

The shell environment fallback path could invoke an attacker-controlled shell when SHELL was inherited from an untrusted host environment. In affected builds, shell-env loading used $SHELL -l -c 'env -0' without validating that SHELL points to a trusted executable.

In threat-model terms, this requires local environment compromise or untrusted startup environment injection first; it is not a remote pre-auth path. The hardening patch validates SHELL as an absolute normalized executable, prefers /etc/shells, applies trusted-prefix fallback checks, and falls back safely to /bin/sh when validation fails. The dangerous env-var policy now also blocks SHELL overrides.