Security and abuse-case tests — T8

One negative test per row of design.md § Security design's abuse-case table (cases 2 to 9; case 1, the unauthenticated client, is the perimeter's by decision-003 and has no harness test). This is also the record the security-review node reads.

## uv run pytest tests/security -q
........................                                                 [100%]
24 passed in 2.88s

Tests by abuse case

Abuse case	Test module
2 — a tool call outside the registry or its schema	test_tool_boundary.py, test_schema_drift.py
3 — a plugin escaping its directory or expanding variables in command	test_plugin_paths.py
4 — a non-member on a channel, a non-participant on a task	test_channel_membership.py, test_ingress.py, test_role_change.py
5 — a remote card requiring what the harness does not implement, or a security scheme	test_remote_card.py
6 — a credential reaching a log, a span, a record or an exception	test_redactor.py, test_push_token.py
7 — an oversized or flooding request	test_oversized_request.py
9 — a forged A2UI action naming a surface the task did not create	test_a2ui_action.py

Collected tests:

test_a2ui_action.py::test_unknown_a2ui_action_discarded
test_channel_membership.py::test_non_member_rejected_without_task_existence
test_ingress.py::test_ingress_redacted_before_history
test_oversized_request.py::test_oversized_request_not_persisted
test_oversized_request.py::test_flooding_peer_is_rate_limited_before_the_route
test_plugin_paths.py::test_plugin_path_escape_rejected
test_plugin_paths.py::test_command_with_expansion_rejected
test_plugin_paths.py::test_only_the_two_variables_expand_and_only_in_args_env_cwd
test_plugin_paths.py::test_contained_rejects_symlink_and_dotdot_escapes
test_plugin_paths.py::test_skill_and_prompt_paths_outside_the_root_are_skipped
test_push_token.py::test_push_token_encrypted_at_rest
test_push_token.py::test_push_key_must_be_base64_of_a_valid_aes_key
test_redactor.py::test_redactor_masks_tokens
test_redactor.py::test_configured_secret_values_are_masked_whatever_their_shape
test_redactor.py::test_short_or_ordinary_text_is_untouched
test_redactor.py::test_scrub_model_recurses_and_keeps_non_strings
test_redactor.py::test_scrub_returns_the_same_instance_when_nothing_changes
test_redactor.py::test_enum_fields_keep_their_type
test_remote_card.py::test_remote_card_with_unknown_required_ext_refused
test_remote_card.py::test_remote_card_with_security_scheme_refused
test_role_change.py::test_role_change_requires_admin
test_schema_drift.py::test_schema_drift_refuses_invoke
test_tool_boundary.py::test_injected_tool_call_not_executed
test_tool_boundary.py::test_unknown_tool_rejected