Compose the two salvaged approaches (#78065 + #78511):
- Keep #78065's terminal-only scrub-path exemption (first-party prefix
predicate in _make_run_env / _sanitize_subprocess_env, plain env values
never scope-resolved, snapshot exclusion for cross-profile isolation,
every non-terminal surface sealed).
- Fold #78511's BUZZ_MANAGED_AGENT signal into a context gate instead of
an import-time blocklist discard: the blocklist is shared by every
scrub surface, so discarding there would leak BUZZ_PRIVATE_KEY into
execute_code / hermes_subprocess_env children too.
- New gate _buzz_terminal_context_active(): BUZZ_MANAGED_AGENT in the
process env (Buzz Desktop buzz-acp harness, #76243) OR the live
session's platform is buzz (HERMES_SESSION_PLATFORM ContextVar,
concurrency-safe under a multi-session gateway). A Telegram/CLI/cron
session on a host that also runs a Buzz gateway does NOT get the
signing key in its terminal children (maintainer triage note on
#76243: don't expose the key to unrelated shell commands).
- Snapshot exclusion stays prefix-only (conservative even when the gate
is inactive).
- Tests updated for the gate + new negative test (non-Buzz session
strips) and positive test (buzz session platform enables carve-out);
docs updated accordingly.
Closes#78026, closes#76243.
Teknium