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.

tools/delegate_tool.py

@@ -35,8 +35,34 @@ DELEGATE_BLOCKED_TOOLS = frozenset([
     "execute_code",    # children should reason step-by-step, not write scripts
 ])
 
-MAX_CONCURRENT_CHILDREN = 3
+_DEFAULT_MAX_CONCURRENT_CHILDREN = 3
 MAX_DEPTH = 2  # parent (0) -> child (1) -> grandchild rejected (2)
+
+
+def _get_max_concurrent_children() -> int:
+    """Read delegation.max_concurrent_children from config, falling back to
+    DELEGATION_MAX_CONCURRENT_CHILDREN env var, then the default (3).
+
+    Uses the same ``_load_config()`` path that the rest of ``delegate_task``
+    uses, keeping config priority consistent (config.yaml > env > default).
+    """
+    cfg = _load_config()
+    val = cfg.get("max_concurrent_children")
+    if val is not None:
+        try:
+            return max(1, int(val))
+        except (TypeError, ValueError):
+            logger.warning(
+                "delegation.max_concurrent_children=%r is not a valid integer; "
+                "using default %d", val, _DEFAULT_MAX_CONCURRENT_CHILDREN,
+            )
+    env_val = os.getenv("DELEGATION_MAX_CONCURRENT_CHILDREN")
+    if env_val:
+        try:
+            return max(1, int(env_val))
+        except (TypeError, ValueError):
+            pass
+    return _DEFAULT_MAX_CONCURRENT_CHILDREN
 DEFAULT_MAX_ITERATIONS = 50
 _HEARTBEAT_INTERVAL = 30  # seconds between parent activity heartbeats during delegation
 DEFAULT_TOOLSETS = ["terminal", "file", "web"]
@@ -600,8 +626,17 @@ def delegate_task(
         return tool_error(str(exc))
 
     # Normalize to task list
+    max_children = _get_max_concurrent_children()
     if tasks and isinstance(tasks, list):
-        task_list = tasks[:MAX_CONCURRENT_CHILDREN]
+        if len(tasks) > max_children:
+            return tool_error(
+                f"Too many tasks: {len(tasks)} provided, but "
+                f"max_concurrent_children is {max_children}. "
+                f"Either reduce the task count, split into multiple "
+                f"delegate_task calls, or increase "
+                f"delegation.max_concurrent_children in config.yaml."
+            )
+        task_list = tasks
     elif goal and isinstance(goal, str) and goal.strip():
         task_list = [{"goal": goal, "context": context, "toolsets": toolsets}]
     else:
@@ -661,7 +696,7 @@ def delegate_task(
         completed_count = 0
         spinner_ref = getattr(parent_agent, '_delegate_spinner', None)
 
-        with ThreadPoolExecutor(max_workers=MAX_CONCURRENT_CHILDREN) as executor:
+        with ThreadPoolExecutor(max_workers=max_children) as executor:
             futures = {}
             for i, t, child in children:
                 future = executor.submit(
@@ -965,9 +1000,11 @@ DELEGATE_TASK_SCHEMA = {
                     },
                     "required": ["goal"],
                 },
-                "maxItems": 3,
+                # No maxItems — the runtime limit is configurable via
+                # delegation.max_concurrent_children (default 3) and
+                # enforced with a clear error in delegate_task().
                 "description": (
-                    "Batch mode: up to 3 tasks to run in parallel. Each gets "
+                    "Batch mode: tasks to run in parallel (limit configurable via delegation.max_concurrent_children, default 3). Each gets "
                     "its own subagent with isolated context and terminal session. "
                     "When provided, top-level goal/context/toolsets are ignored."
                 ),