Subscribe to live governance signal updates (SSE)

Server-Sent Events stream that emits one event: signal per governance_signals insert/update matching the caller's filter. For scope=org|agent it ALSO emits an event: agent.claimed frame ({agent_id, org_id, name, claimed_at}) when an agent is claimed into the scope, so an open dashboard shows it appear live (Dojo MNE-521). RBAC mirrors the REST list endpoints: scope=platform requires Mnemom staff; scope=org|team|agent requires membership of the org that owns the named scope id: scope=org needs org_id, scope=team needs team_id and scope=agent needs agent_id (400 without it), and every non-platform stream carries only rows of that one org. The stream emits an initial event: ready marker, then events as they arrive, plus periodic :heartbeat comments. Connection lifetime is capped at 5 minutes — clients should reconnect.