fix(tests): resolve 10 CI failures across hooks, tiktoken, plugins (#3848)

test_hooks.py (7 failures): Built-in boot-md hook was always loaded
by _register_builtin_hooks(), adding +1 to every expected hook count.
Mock out built-in registration in TestDiscoverAndLoad so tests isolate
user-hook discovery logic.

test_tool_token_estimation.py (2 failures): tiktoken is not in
core/[all] dependencies. The estimation function gracefully returns {}
when tiktoken is missing, but tests expected non-empty results. Added
skipif markers for tests that need tiktoken.

test_plugins_cmd.py (1 failure): bare 'hermes plugins' now dispatches
to cmd_toggle() (interactive curses UI) instead of cmd_list(). Updated
test to match the new behavior.

tests/test_plugins_cmd.py

@@ -150,11 +150,11 @@ class TestPluginsCommandDispatch:
         plugins_command(args)
         mock_list.assert_called_once()
 
-    @patch("hermes_cli.plugins_cmd.cmd_list")
-    def test_none_falls_through_to_list(self, mock_list):
+    @patch("hermes_cli.plugins_cmd.cmd_toggle")
+    def test_none_falls_through_to_toggle(self, mock_toggle):
         args = self._make_args(None)
         plugins_command(args)
-        mock_list.assert_called_once()
+        mock_toggle.assert_called_once()
 
     @patch("hermes_cli.plugins_cmd.cmd_install")
     def test_install_dispatches(self, mock_install):