refactor(delegate): drop dead default_toolsets from CLI default config

delegation.default_toolsets was declared in cli.py's CLI_CONFIG default
dict and documented in cli-config.yaml.example, but never read: none of
tools/delegate_tool.py, _load_config(), or any call site ever looked it
up. The live fallback is the DEFAULT_TOOLSETS module constant at
tools/delegate_tool.py:101, which stays as-is.

hermes_cli/config.py's DEFAULT_CONFIG["delegation"] already omits the
key — this commit aligns cli.py with that.

Adds a regression test in tests/hermes_cli/test_config_drift.py so a
future refactor that re-adds the key without wiring it up to
_load_config() fails loudly.

Part of Initiative 2 / M0.5.

tests/hermes_cli/test_config_drift.py

@@ -0,0 +1,25 @@
+"""Regression tests for removed dead config keys.
+
+This file guards against accidental re-introduction of config keys that were
+documented or declared at some point but never actually wired up to read code.
+Future dead-config regressions can accumulate here.
+"""
+
+
+def test_delegation_default_toolsets_removed_from_cli_config():
+    """delegation.default_toolsets was dead config — never read by
+    _load_config() or anywhere else. Removed in M0.5.
+
+    Guards against accidental re-introduction in cli.py's CLI_CONFIG default
+    dict. If this test fails, someone re-added the key without wiring it up
+    to _load_config() in tools/delegate_tool.py.
+    """
+    from cli import CLI_CONFIG
+
+    delegation_cfg = CLI_CONFIG.get("delegation", {})
+    assert "default_toolsets" not in delegation_cfg, (
+        "delegation.default_toolsets was removed in M0.5 because it was "
+        "never read. Do not re-add it; use tools/delegate_tool.py's "
+        "DEFAULT_TOOLSETS module constant or wire a new config key through "
+        "_load_config()."
+    )