M4: task tool, subagent spawner, board wiring

Adds the task tool and subagent spawner in harness-tools, and wires spawning/aliasing/reuse and depth limits through harness-app and the job board, so an orchestrator session can launch and track child sessions.

crates/harness-tools/src/bash.rs

@@ -149,6 +149,7 @@ mod tests {
                 CancellationToken::new(),
             ),
             metadata,
+            spawner: None,
         }
     }