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/prompts.rs

@@ -84,13 +84,13 @@ pub(in crate::config) const DEFAULT_SPAWN_INSTRUCTIONS: &str = indoc! {"
 
     | Tool | Purpose |
     |------|----------|
-    | `agent__spawn` | Spawn a subagent in the background. Returns an `id` immediately. |
+    | `agent__spawn` | Spawn a subagent in the background. Returns an `id` immediately. Optional `variables` (string map) sets the target agent's declared variables, overriding inherited values. |
     | `agent__check` | Non-blocking status probe: running or finished. Never returns/consumes the result — use `agent__collect`. |
     | `agent__collect` | Blocking wait: wait for an agent to finish, return its output. |
     | `agent__list_available` | List all agent types you can spawn (name + description). Use this to discover specialists before calling `agent__spawn`. |
     | `agent__list_running` | List all subagents YOU have spawned, with their status. |
     | `agent__cancel` | Cancel a running agent by ID. |
-    | `agent__task_create` | Create a task in the dependency-aware task queue. |
+    | `agent__task_create` | Create a task in the dependency-aware task queue. Optional `variables` (string map) is passed to the auto-dispatched agent. |
     | `agent__task_list` | List all tasks and their status/dependencies. |
     | `agent__task_complete` | Mark a task done; returns any newly unblocked tasks. Auto-dispatches agents for tasks with a designated agent. |
     | `agent__task_fail` | Mark a task as failed. Dependents remain blocked. |