Extension proposal: signed-receipts/v1 — a did:web key-trust convention for §8.4 + signed task-outcome receipts

§8.4 standardises the AgentCard JWS envelope but deliberately leaves the key-trust root unspecified, and attestation of task *outcomes* is unstandardised — two agents can interoperate, but neither can hand a third party durable evidence of what happened.

We've drafted an extension (experimental tier) that adds:
1. a key-trust convention: JWS `kid` as a `did:web` URL, resolved at `/.well-known/did.json` — no new registry or PKI;
2. a signed receipt object (RFC 8785 canonical JSON, Ed25519, offline-verifiable) an agent MAY attach to task completion via `Task.metadata`;
3. a normative register: a receipt is evidence of what was claimed and when — never a certification or endorsement.

Spec + ~100-line reference interceptor (Apache-2.0, roundtrip/tamper-tested): https://github.com/CSOAI-ORG/a2a-signed-receipts

Is the experimental extension path the right venue for this? Happy to adapt to the extension governance process and any naming/shape feedback.