feat(agents): allow passing agent variables to agent__spawn and agent__task_create

- agent__spawn accepts an optional variables object (string->string);
  values overlay the parent-inherited agent variables per key, with
  inheritance preserved as the fallback layer
- unknown variable names, non-string values, and non-object shapes
  return self-correcting status:error JSON listing the agent's
  declared variables
- spawned children can no longer trigger the interactive variable
  prompt: a new interactive gate in Agent::init_agent_variables routes
  missing required variables to the existing bail path while leaving
  CLI/REPL prompting and --info skip semantics unchanged
- agent__task_create persists variables on the task, agent__task_list
  echoes them, and task auto-dispatch threads them into the spawn
- tool schemas and built-in spawn docs updated; unit tests extended
  in agents.rs and taskqueue.rs

src/supervisor/mod.rs

@@ -504,7 +504,7 @@ mod tests {
         let mut sup = Supervisor::new(4, 3);
         let id = sup
             .task_queue_mut()
-            .create("task".into(), "desc".into(), None, None);
+            .create("task".into(), "desc".into(), None, None, None);
         assert!(!id.is_empty());
         assert_eq!(sup.task_queue().list().len(), 1);
     }