harness: add request_next_turn MCP tool — immediate self-continuation (closes #216)

hive-ag3nt/src/mcp.rs

@@ -626,6 +626,30 @@ impl AgentServer {
         })
         .await
     }
+
+    #[tool(
+        description = "Ask the harness to start another turn immediately after this one \
+            completes, even if the inbox is empty. Use this when you have ongoing work that \
+            spans multiple turns (long builds, multi-step tasks) and you want to continue \
+            without waiting for an external message. The next turn will start with \
+            `from: \"self\"` and `body: \"continue\"`. Has no effect if a new inbox message \
+            arrives before this turn ends — the harness already loops immediately on pending \
+            messages. No args."
+    )]
+    async fn request_next_turn(&self) -> String {
+        run_tool_envelope("request_next_turn", String::new(), async move {
+            let sentinel = crate::paths::state_dir().join("hyperhive-continue");
+            match std::fs::write(&sentinel, b"") {
+                Ok(()) => "ok — harness will start another turn immediately after this one",
+                Err(e) => {
+                    tracing::warn!(error = %e, path = %sentinel.display(), "request_next_turn: write failed");
+                    return format!("request_next_turn failed: {e}");
+                }
+            }
+            .to_string()
+        })
+        .await
+    }
 }
 
 #[tool_handler(