feat: implement initial supervisor middleware #2010

Description

Deliver the first usable version of supervisor middleware, establishing the shared contract and runtime for data-plane extensions through an outbound HTTP request hook.

This work landed through PR #2027.

Delivered Scope

• The openshell.middleware.v1 protobuf contract for HTTP_REQUEST/PRE_CREDENTIALS.
• Built-in middleware that runs inside the supervisor and statically registered operator-run gRPC middleware services.
• Gateway registration, manifest discovery, policy validation, and delivery of effective middleware configuration to supervisors.
• Policy-selected, ordered middleware chains using destination host selectors.
• Allow, deny, request-body transformation, safe header mutation, findings, metadata, and explicit fail-open or fail-closed behavior.
• Request inspection after network and L7 policy admission and before OpenShell-managed credential injection.
• Body limits, timeouts, last-known-good registry behavior, and audit-safe OCSF events.
• The built-in openshell/regex redactor and an external content-guard example.
• User-facing policy, gateway configuration, operations, and extensibility documentation.

Boundaries

The initial hook covers inspectable HTTP/1.x requests and WebSocket upgrade requests. Post-upgrade WebSocket messages, HTTP/2 and gRPC streams, credential-visible request hooks, response processing, additional deployment models, authenticated middleware transport, and runtime management are separate follow-up capabilities.

References

• Parent capability: Supervisor middleware #1733
• Design: RFC 0009 in rfc/0009-supervisor-middleware/
• Implementation: PR feat(supervisor-middleware): add network egress middleware #2027
• Example middleware: PR feat(examples): add supervisor middleware content guard #2169

Definition of Done

• Define the v1 middleware service and message contract.
• Preserve middleware configuration through policy YAML and protobuf round trips.
• Validate registrations, manifests, policy attachments, failure behavior, and uninspectable endpoint conflicts.
• Execute built-in and operator-run middleware chains in the supervisor request path.
• Enforce safe transformations before credential injection and re-evaluate body-aware L7 policy after transformation.
• Emit audit-safe OCSF events for decisions, failures, transformations, and findings.
• Cover the contract, policy, registry, relay, and external gRPC paths with automated tests.
• Document configuration and operation of supervisor middleware.