CVE-2026-53951: Copier has a trust-prefix bypass via path traversal that runs tasks unprompted

In copier >= 9.5.0, <= 9.15.1, the trust setting’s prefix match (copier/_settings.py) compares the template URL against a trusted prefix with a raw str.startswith and no path normalization, while the URL is normalized when the template is actually fetched (Path.resolve() for local paths; libcurl dot-segment removal for https). A template reference that textually starts with a trusted prefix but contains .. (e.g. https://github.com/trusted-org/../attacker-org/repo.git) is therefore granted trust yet resolves to a different, attacker-controlled template, whose tasks / migrations / jinja_extensions then run without the --trust prompt — arbitrary command execution. Likely CWE-22 (Improper Limitation of a Pathname) in the trust check leading to CWE-94 (code execution).