Salvaged premise from #67065 (@webtecnica, issue #67027), reimplemented:
get_env_value() read os.environ first with no secret-scope check, so a
multiplexed profile turn could serve another profile's credential. Its
siblings get_env_value_prefer_dotenv and gateway.config._getenv were
already scope-aware.
Reimplementation note: the original diff called get_secret() but fell
through to os.environ on a scoped miss — re-opening the exact leak it
targeted (flagged by the sweeper review). This version delegates policy
fully to agent.secret_scope.get_secret (global vars pass through; scope
authoritative under multiplexing; legacy environ behavior when off;
UnscopedSecretError propagates fail-closed), then falls back to .env.
6 regression tests incl. the #67027 repro (envless profile + multiplexed
turn -> None, not the other profile's key); sabotage-verified RED on the
old implementation.
webtecnica