Attestation verification bypass via unescaped SAN regex

Incorrect regular expression handling in GitHub CLI before 2.97.0 allows attackers to bypass Sigstore attestation signer checks. The gh attestation verify policy code interpolates --signer-repo and --signer-workflow values into certificate SAN patterns without escaping metacharacters, so a dot matches a different character in a lookalike signer path. Exploitation requires a valid attestation from a plausible lookalike repository and a verifier that pins trust with either affected flag, allowing untrusted artifacts to satisfy that policy.