Follow-up mitigation for the #27303 env-scrub tightening. Dropping the
broad HERMES_ prefix in favor of a 4-var operational allowlist is correct
hardening, but a sandbox script that imports a repo module reading a
non-allowlisted HERMES_* var at import time would otherwise see it
silently unset. _scrub_child_env now emits a one-shot debug log naming the
dropped non-secret HERMES_* vars and pointing at the env_passthrough
opt-in escape hatch. Secret-shaped vars are never named in the log.
Tests: dropped vars are logged + env_passthrough named; no log when
nothing is dropped; secret vars excluded from the diagnostic.
Cover context+callback propagation and teardown-clears, a source guard that both RPC threads stay wrapped, the check_execute_code_guard decision matrix (isolated backend, headless-local, cron-deny, gateway approve/deny/timeout/missing-notify, smart mode, session-yolo), the env-scrub allowlist/secret rules, and a behavioral test that execute_code() blocks before spawning on denial.
Refs #4146, #27303, #30882, #33057
