docs(get_agent_meta): add matrix_accounts to tool description

The feat added matrix_accounts to the GetAgentMeta response in
commit 5b442796 but didn't update the MCP tool description string
that agents see when introspecting the tool. Add a brief note so
agents know to expect the field.

hive-ag3nt/src/mcp.rs

@@ -968,7 +968,10 @@ impl AgentServer {
         identity stamp — handy for state files / commit messages / cross-agent \
         attribution that won't drift across renames or session-continue boundaries \
         where the system-prompt label could be stale. Status reads `<none>` when the \
-        target has never called `set_status` or has cleared it."
+        target has never called `set_status` or has cleared it. Also returns \
+        `matrix_accounts` — a list of matrix identities the agent can act as \
+        (`name`, `user_id?`, `homeserver`); omitted for agents with no matrix \
+        provisioning."
     )]
     async fn get_agent_meta(&self, Parameters(args): Parameters<GetAgentMetaArgs>) -> String {
         let log = args.name.clone().unwrap_or_else(|| "<self>".to_owned());