Real-time decision callback (Flow C / L3)

[Faux16]

Context

The sdk-python README lists this under "Does NOT implement yet (Phase 1+)":

Real-time decision callback (Flow C)

Flow C is the L3 synchronous decision callback — vendor pauses an action, asks the plane "is this allowed?", and waits for a signed decision before proceeding. Today the SDK can produce and verify decision requests/responses (the schemas are bundled) but doesn't have a high-level client for actually doing the round-trip.

Goal

Ship openagp.flow_c (or similar) with:

• Vendor-side client. Sends a DecisionRequest, waits for a signed DecisionResponse, verifies it, surfaces it to the caller.
• Plane-side server hook. Receives a DecisionRequest, calls a user-supplied decision function (probably reusing the Phase β policy evaluator), produces a signed DecisionResponse.
• Timeout + retry semantics — clearly documented; a slow plane is its own failure mode and the vendor needs to decide fail-open vs. fail-closed per spec.

Scope

• Sync API surface (don't force async on apps that don't need it).
• Built on top of the HTTP scaffolds (related issue) — landing in the same release.
• Test vectors should be added to openagp/spec/test-vectors/ for byte-identical cross-SDK parity.

Pointers

• Decision schemas: openagp/spec/schemas/decision-request.json, decision-response.json
• Sibling TS issue: realtime-decision in openagp/sdk-typescript
• Example tracking issue: realtime-decision in openagp/examples