feat: sandbox egress middleware extension hooks #1906

Description

Track the extensibility capability proposed by RFC 0005: a supervisor egress middleware hook that lets OpenShell call trusted external middleware before outbound sandbox requests leave the sandbox boundary.

At a high level, this gives OpenShell a policy-selected extension point where middleware can inspect request content, transform or redact it, deny it, and attach metadata or audit annotations while OpenShell remains the enforcement boundary.

Context

• RFC PR: rfc-0009: supervisor middleware #1738
• Existing RFC tracking issue: Supervisor middleware #1733
• Parent extensibility roadmap: OpenShell Extensibility #1904

What This Unlocks

• Lets OpenShell support value-add controls such as Privacy Guard without baking every guard directly into the core supervisor.
• Gives operators a place to plug in domain-specific request governance for outbound sandbox traffic.
• Creates a foundation for future middleware integrations around L7 inspection, redaction, request annotation, and audit evidence.
• Keeps the core product focused on sandbox enforcement while allowing optional external services to extend behavior.

Definition of Done

• RFC accepted or closed with a clear disposition.
• Follow-on implementation issues are created for the accepted v1 scope.
• Child work is linked under the relevant roadmap/project item so project tracking stays accurate.