fix(delegate): guard _load_config() against delegation: null in config.yaml

[ideathinklab01-source]

Problem

delegation: null in config.yaml causes NoneType object has no attribute 'get' on every API call that triggers the delegation path:

File "run_agent.py", line 11938, in run_conversation
    assistant_message.tool_calls = self._cap_delegate_task_calls(
File "run_agent.py", line 4733, in _cap_delegate_task_calls
    max_children = _get_max_concurrent_children()
File "tools/delegate_tool.py", line 335, in _get_max_concurrent_children
    val = cfg.get("max_concurrent_children")
          ^^^^^^^
AttributeError: 'NoneType' object has no attribute 'get'

Root Cause

_load_config() uses dict.get("delegation", {}) which returns the default {} only when the key is missing. When delegation: null is present in YAML, the key exists with value None, so .get() returns None — and downstream cfg.get(...) crashes.

This is the same class of bug as the one fixed in fd9b692 (fix(tui): tolerate null top-level sections in config.yaml), which guarded 21 sites in tui_gateway/server.py but missed delegate_tool.py.

Related issues: #7215, #7346

Fix

Use dict.get(key) or {} instead of dict.get(key, {}) in _load_config(), consistent with the pattern from fd9b692:

# Before
cfg = CLI_CONFIG.get("delegation", {})
return full.get("delegation", {})

# After
cfg = CLI_CONFIG.get("delegation") or {}
return full.get("delegation") or {}

Testing

• Verified that delegation: null no longer crashes; _get_max_concurrent_children() correctly falls back to the default (3)
• Verified that delegation: {} and delegation: {max_concurrent_children: 5} still work as expected

[teknium1]

Salvaged via #19662 onto current main - your commit authorship was preserved. Thanks!