Whitelist mode now clears the parent env and re-adds a small allowlist
(identity, terminal, locale, proxy, non-GUI XDG, vendor prefixes).
Blacklist mode strips cloud credentials, backup passphrases, dangling
socket pointers, and anything matching *_TOKEN, *_SECRET, *_PASSWORD,
*_PASSPHRASE, *_API_KEY, *_PRIVATE_KEY, *_CLIENT_SECRET; vendor prefix
carve-outs keep ANTHROPIC_API_KEY and friends.
Users can override via --setenv KEY=VALUE and --unsetenv KEY (and the
corresponding TOML keys), or opt out of the built-in policy entirely
with --no-env-filter.
