fix: restore side-effect imports in test files (test_kanban_tools, test_command_guards)

The previous ruff prune commit removed two categories of test-file
imports whose value is the side effect of importing them, not their
binding:

  tests/tools/test_kanban_tools.py — 5 sites
    `import tools.kanban_tools  # ensure registered`
    The import itself runs tools/kanban_tools.py's @registry.register
    calls; without it, the kanban tool registry is empty and
    test_kanban_tools_visible_with_env_var asserts {} != {7 kanban tools}.

  tests/tools/test_command_guards.py — 1 site
    `import tools.tirith_security  # Ensure the module is importable so we can patch it`
    The comment names the requirement: keep the bare module reference
    so subsequent mock.patch("tools.tirith_security.<fn>") calls find
    a registered submodule.

CI failure: test (5) shard, tests/tools/test_kanban_tools.py:58
  AssertionError: expected {kanban_*}, got set()

tests/tools/test_command_guards.py

@@ -10,9 +10,12 @@ from tools.approval import (
     approve_session,
     check_all_command_guards,
     is_approved,
+    set_current_session_key,
+    reset_current_session_key,
 )
 
 # Ensure the module is importable so we can patch it
+import tools.tirith_security
 
 
 # ---------------------------------------------------------------------------