docs(readme): document peer_name/peer_role/agent_card_url envelope fields (2026-05-02)

The platform's inbound A2A envelope was enriched on 2026-05-02 with
three additional meta fields that the README's "How replies work"
sample didn't cover:

- peer_name      — peer's display name (registry-resolved; may be absent
                   on registry-lookup failure)
- peer_role      — peer's declared role (same registry source)
- agent_card_url — deterministic URL of the platform's discover endpoint
                   for this peer (always populated)

Updates the JSON sample to include all three (existing fields retained,
nothing removed) and adds a one-line compatibility note dated to the
change with a forward-pointer to the runtime-mcp docs page.

The plugin already forwards whatever the platform sends, so older
platforms that pre-date this change continue to work unchanged — only
the docs were stale.

README.md

@@ -87,6 +87,9 @@ When a peer's message lands in your session, the meta block carries the routing
       "workspace_id": "ws-uuid-1",
       "watching_as": "ws-uuid-1",
       "peer_id": "ws-uuid-pm-coordinator",
+      "peer_name": "ops-agent",
+      "peer_role": "sre",
+      "agent_card_url": "https://your-tenant.staging.moleculesai.app/registry/discover/ws-uuid-pm-coordinator",
       "method": "user_message",
       "activity_id": "act-...",
       "ts": "2026-04-29T..."
@@ -95,6 +98,8 @@ When a peer's message lands in your session, the meta block carries the routing
 }
 ```
 
+> **Compatibility note (2026-05-02):** the platform now enriches the inbound envelope with three additional fields — `peer_name` (peer's display name, registry-resolved), `peer_role` (peer's declared role, same registry source), and `agent_card_url` (deterministic URL of the platform's discover endpoint for this peer). `peer_name` and `peer_role` may be absent when the registry lookup fails (e.g. the peer hasn't registered yet); `agent_card_url` is always populated because it's computed deterministically from `peer_id`. Pre-2026-05-02 platforms do not emit these fields — the plugin forwards whatever the platform sends, so older payloads continue to work unchanged. See <https://doc.moleculesai.app/docs/runtime-mcp> for the full envelope spec.
+
 Claude can call `reply_to_workspace({peer_id, text})` to send the response back. If only one workspace is watched, `workspace_id` is implicit. Multi-workspace setups need the watched id explicitly.
 
 ## Architecture notes