fix(delegate): make max_concurrent_children configurable + error on excess

`delegate_task` silently truncated batch tasks to 3 — the model sends
5 tasks, gets results for 3, never told 2 were dropped. Now returns a
clear tool_error explaining the limit and how to fix it.

The limit is configurable via:
  - delegation.max_concurrent_children in config.yaml (priority 1)
  - DELEGATION_MAX_CONCURRENT_CHILDREN env var (priority 2)
  - default: 3

Uses the same _load_config() path as the rest of delegate_task for
consistent config priority. Clamps to min 1, warns on non-integer
config values.

Also removes the hardcoded maxItems: 3 from the JSON schema — the
schema was blocking the model from even attempting >3 tasks before
the runtime check could fire. The runtime check gives a much more
actionable error message.

Backwards compatible: default remains 3, existing configs unchanged.

tests/run_agent/test_agent_guardrails.py

@@ -9,7 +9,9 @@ Covers three static methods on AIAgent (inspired by PR #1321 — @alireza78a):
 import types
 
 from run_agent import AIAgent
-from tools.delegate_tool import MAX_CONCURRENT_CHILDREN
+from tools.delegate_tool import _get_max_concurrent_children
+
+MAX_CONCURRENT_CHILDREN = _get_max_concurrent_children()
 
 
 # ---------------------------------------------------------------------------