Proposal: experimental A2A extension for offline delivery / reachability (Agent Card discovery)

## Abstract

Propose an experimental A2A extension that advertises **offline delivery /
reachability** support on the Agent Card: when an agent’s effective mode is
store-and-forward (`sleep` / `hourly_wake`), peers can discover that profile
without inventing incompatible card params.

## Motivation

Core A2A does not define Agent Card vocabulary for asleep / store-and-forward
reachability. Implementations that queue while offline today either hide that
fact or invent one-off card fields. An optional, card-only extension lets
adopters advertise a shared profile whose **wire** semantics live in a separate
Internet-Draft.

## Technical approach

- **Current identifier (self-service):** `https://atom.qwixl.dev/a2a/offline-delivery/v1`
- **Card:** `required: false`; `params.mode` is `sleep` or `hourly_wake` (effective mode at card issue)
- **Scope:** Agent Card discovery only — no media type; MUST NOT require stamping the URI into `message.extensions` / `A2A-Extensions`
- **Wire:** [`draft-chapman-a2a-offline-delivery-00`](https://datatracker.ietf.org/doc/draft-chapman-a2a-offline-delivery/) owns asleep / queue / validate-on-dequeue semantics; live HTTP responses remain authoritative over a stale card
- **Spec + schemas + examples:** https://github.com/Qwixl/Atom/tree/main/spec/extensions/offline-delivery-v1
- **Conformance:** `spec/vectors/offline-delivery/` (34 vectors) + CI
- **Reference implementation:** `@qwixl/agent-backend` / `@qwixl/a2a-transport` (Apache-2.0)
- **Document host:** SPEC in the GitHub tree above; the URI is an opaque identifier (HTTP resolution at `atom.qwixl.dev` may follow later)

MLS, rooms, Governed Objects, and push vendors are **explicitly out of scope**
for this extension.

## Ask

Looking for a Maintainer sponsor toward an experimental offline-delivery /
reachability extension under `a2aproject`, per
https://a2a-protocol.org/latest/topics/extension-and-binding-governance/

Happy to revise naming, URI migration to `a2a-protocol.org/extensions/…` if
and when an experimental repo graduates, and packaging to match other
experimental extensions.

Review the spec and examples at spec/extensions/offline-delivery-v1 in the repository. Understand the draft-chapman-a2a-offline-delivery IETF draft for wire semantics. Examine the reference implementation in @qwixl/agent-backend and @qwixl/a2a-transport. Run the conformance tests in spec/vectors/offline-delivery/ to verify the extension's behavior.

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