agent/secret_scope.py has pointed at docs/design/multiplexing-gateway.md
("Workstream A") since the fail-closed secret scope landed, but the file was
never added. This writes the missing doc from the code as it stands today:
the mode flag, scope composition (_profile_runtime_scope seams), the
context-local secret scope and HERMES_HOME override, routing/serving/
persistence/session-lane isolation, the control-plane RPCs, failure modes,
and an honest table of what is still process-global (per-profile MCP
registries are tracked in #67605).
Doc-only change; no code touched. Style follows docs/profile-routing.md.
Teknium