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/config/agent.rs

@@ -329,6 +329,7 @@ impl Agent {
         agent_variables: &[AgentVariable],
         pre_set_variables: Option<&AgentVariables>,
         no_interaction: bool,
+        interactive: bool,
     ) -> Result<AgentVariables> {
         let mut output = IndexMap::new();
         if agent_variables.is_empty() {
@@ -349,7 +350,7 @@ impl Agent {
             if no_interaction {
                 continue;
             }
-            if *IS_STDOUT_TERMINAL {
+            if interactive && *IS_STDOUT_TERMINAL {
                 if !printed {
                     println!("⚙ Init agent variables...");
                     printed = true;