Build `mock-plane` example (L1 HTTP service receiving + verifying)

[Faux16]

Context

The examples README lists this as coming:

mock-plane/ — Minimal HTTP service receiving and verifying events (L1)

Symmetric counterpart to mock-vendor.

Goal

A self-contained example under examples/mock-plane/ that:

1. Runs as a FastAPI HTTP service.
2. Exposes a POST /events endpoint accepting signed AGP events.
3. Resolves the signing key (config-driven trust store for now; registry-driven once the resolver ships).
4. Verifies signature, validates schema, stores the event in an append-only in-memory ledger.
5. Exposes GET /events for the operator to inspect what's been received.

Acceptance

• make demo from examples/mock-plane/ boots the service.
• Pointing mock-vendor at it produces verifiable events on the plane side.
• agp-cts validate-plane --endpoint http://localhost:PORT (once that command lands — tracked in cts) returns "conformant L1".
• Smoke test in tests/ posts a well-formed event (accepted) and a tampered event (rejected with a clear error).
• README states: L1 only, no policy delivery / no Flow C.

Scope

• Built on the Python SDK HTTP scaffold (related issue).
• In-memory storage only; persistence is out of scope.
• Replay-cache integration (once that ships — tracked in sdk-python) — wire the hook even if cache is a no-op stub for now.

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🚀 Start here

What: a minimal HTTP service that receives AGP events and verifies their signatures (L1). The counterpart to mock-vendor (#2).

Files: examples/mock-plane/ — README.md, Makefile (demo target), the service.

How to build it:

from openagp.events import verify          # pip install openagp
# on POST /agp/v0/events: verify(event, public_key_b64=...) ; 202 on success, 400 on bad signature

Acceptance criteria:

• Accepts events, verifies signatures, rejects tampered ones (flip a byte → rejected).
• make demo shows a good event accepted and a bad one rejected.
• README explains verification + key resolution; use the SDK.
• DCO sign-off.