feat(#2300): recv reports remaining inbox depth so agents know how many messages are left

hive-ag3nt/src/mcp/mod.rs

@@ -248,6 +248,9 @@ impl AgentServer {
             any time you expect a burst — one tool call beats N consecutive single recvs. \
             `wait_seconds` still applies to the FIRST message; once one arrives the call drains \
             up to `max` in total. Empty result reported the same way regardless of `max`. \n\n\
+            After popping, the result appends a `(N more message(s) pending …)` line whenever the \
+            inbox still has queued messages — so you know whether to drain again (or `ack_until`) \
+            without a separate status check. No line means the inbox is empty. \n\n\
             Typical pattern: when you have nothing else useful to do, call \
             `recv(wait_seconds: 180)` to park until something arrives."
     )]