Add portable agent-request/v1 offline core

[JRichlen]

Public agent workflows lacked a portable request-time contract that keeps routing labels separate from tool authority. This draft adds an offline agent-request/v1 core with a closed schema, deterministic validation against separately supplied trusted context, exact approval and replay checks, bounded canonical HTTP/OpenAI header mapping, counterfeit fixtures, and safe planner/implementer/reviewer/publisher examples.

The sender rejects unknown or free-form fields before encoding. The receiver rejects duplicate or noncanonical JSON, binds operational fields and dispatcher provenance to trusted context, and returns stable non-sensitive errors for malformed requests and offline fixtures.

Source identity: 6919ed307d79be24bec67b633a4515f38125cfce.

Validation:

• Author: 22 focused tests and 11 review-derived regression cells passed; repository cheap tier reported 1,202 passed, 0 failed.
• Independent final review: 27 negative and 5 positive cells passed; the same 22 focused tests and 11 regression cells passed. Its broad runner reported 1,201 passed, 0 failed because Node was unavailable and one accepted skip-control replaced the two JavaScript route/step checks; those checks did run in the author’s 1,202-test result.

This is an offline core and draft for review. It does not provide a live issuer, signatures, durable replay storage, verified Aperture integration, permissions, gateway deployment, or rollout.

Tracks #101. Related: #84, #88.