feat(#1990): surface pending recv() message count in get_loose_ends

hive-sh4re/src/lib.rs

@@ -386,6 +386,15 @@ pub enum LooseEnd {
         due_at: i64,
         age_seconds: u64,
     },
+    /// Undelivered inbox messages waiting to be `recv`'d by this agent.
+    /// Not cancellable — drain them with `recv`. Surfaced so an agent
+    /// doing a between-turns `get_loose_ends` sweep sees it still owes
+    /// itself a `recv` without having to poll the inbox separately. Only
+    /// emitted when `count > 0`.
+    PendingMessages {
+        /// Number of undelivered messages queued for this agent.
+        count: u64,
+    },
     /// Unread matrix notifications in one or more rooms. Not cancellable —
     /// use `mark_read` via the matrix MCP to clear. Injected by the
     /// in-container harness (not hive-c0re) because the matrix daemon