test: full-suite runs sample the live session, reddening 6 test_mind_utils tests

[adrianwedd]

The 6 long-standing test_mind_utils failures on the robot are not flaky and
not folklore. They are a test-isolation defect with a confirmed mechanism.

Mechanism

tests/conftest.py has three autouse fixtures that redirect live state to tmp:

line	fixture
12	_isolate_health_writes
35	_isolate_brain_mailbox
55	_isolate_alive_heartbeat

There is no autouse fixture for the session. PX_SESSION_PATH is set on
exactly one line (87), inside isolated_project — which is opt-in, and per
CLAUDE.md only isolates subprocesses. tests/test_mind_utils.py uses
isolated_project zero times.

So any in-process test reaching load_session() reads the live robot's
state/session.json. That file currently has spark_quiet_mode: true (see #209
— an unattributable latch we deliberately are not clearing). policy.py:118 is
the sole reader, and mind.expression() is a #174 enforcement point, so the
dispatch is blocked before any mock fires:

expression: requested=look_around verdict=blocked reason=quiet_mode substituted=none

mock_us.call_args is then None, and the assertion dies on
AttributeError: 'NoneType' object has no attribute 'kwargs'.

Affected tests

• test_expression_play_sound_calls_tool
• test_expression_look_around_calls_tool
• test_expression_time_check_calls_tool
• test_expression_calendar_check_calls_tool
• test_unknown_action_logged
• test_expression_speech_actions_have_no_outcome_key

Evidence it is pre-existing, not caused by #206

Same six tests, same pinned PX_SESSION_PATH (a copy of the live session, so
the only variable is the code):

tree	result
fix/audio-sink-policy-gate	6 failed
origin/master @ 585175e	6 failed

Also note these pass on any machine whose state/session.json has quiet mode
off, or where the file is absent — which is why CI never sees them and why they
have been carried as a known-red baseline.

Proposed fix

A fourth autouse fixture in tests/conftest.py, matching the shape of the
existing three, pointing PX_SESSION_PATH at tmp_path. Tests that mean to
exercise a populated session set it themselves, exactly as the health and brain
tests do today.

This generalises past test_mind_utils: it closes the whole class, rather than
patching six call sites.

Non-goals

• Do not clear the live spark_quiet_mode latch (state(quiet): make quiet mode attributable, bounded and explainable #209).
• Not a fix for the separate load-flakes (test_api, test_brain,
  test_gpio_lease, test_px_alive), which pass in isolation.