test(hooks): guard PreToolUse gate shim against predictable temp file (#384)

The symlink-attack vector (predictable /tmp/cbm-code-discovery-gate-$PPID)
was removed in c29e6d5 when the blocking bash gate became a stateless
compiled augmenter writing to stdout only. Add a regression guard so a
predictable temp/state file can never be reintroduced: assert the emitted
gate shim contains no /tmp path and no $PPID, and delegates to hook-augment.

Export cbm_install_hook_gate_script (was static) so the security property
is directly testable.