Record working files written outside the scratch directory

Makes a Phase 2 null result interpretable rather than ambiguous. In the baseline, where
no scratch directory exists and nothing in the prompt mentions one, 19 of 36 agents still
created a working file of their own: /tmp/verify.py, /tmp/brute.py, /tmp/proto.py, and in
two cases scratch.py beside their work.

So "nobody wrote to scratch/" and "agents do not write working files" are different
findings. Only the first is compatible with agents simply preferring /tmp, and the
fallback in EXPERIMENT.md is the right response to one and not the other. Without this
signal the pilot cannot tell them apart.

Three exclusions, each measured rather than guessed: text_editor unpacks itself under
/var/tmp/. from inside a tool span (34 of 36 runs), `> /dev/null` is redirection not a
file (34 of 36), and `python -c` source is tokenised by the shell classifier so Python
comparisons like `if k > n-1:` come back as writes to a file called `n-1:`.

Claude-Session: https://claude.ai/code/session_01Cq98H7sNoSJdL3W98f18bu

tests/test_checks.py

@@ -126,3 +126,65 @@ def test_written_content_ignores_writes_outside_the_directory() -> None:
     ]
     got = written_content(interactions_from_events(events, spec=SPEC))
     assert got == [("scratch/n.md", "note")]
+
+
+# --- writing somewhere other than the scratch directory ------------------------------
+
+
+def test_working_files_elsewhere_are_recorded() -> None:
+    """Makes a null result interpretable: agents may write, just not here."""
+    use = use_of("bash --login -c \"cat > /tmp/brute.py <<'EOF'\nx=1\nEOF\"")
+    assert use.wrote_elsewhere
+    assert use.elsewhere_paths == ["/tmp/brute.py"]
+    assert not use.wrote and not use.touched
+
+
+def test_the_task_files_are_not_working_files() -> None:
+    """Editing func.py is the task, not the agent keeping notes."""
+    use = use_of(
+        "bash --login -c \"echo x > func.py\"",
+        "bash --login -c \"echo y > /workspace/test.py\"",
+    )
+    assert not use.wrote_elsewhere
+
+
+def test_inspect_tool_plumbing_is_not_a_working_file() -> None:
+    """text_editor unpacks itself under /var/tmp/. from inside a tool span.
+
+    It appeared in 34 of 36 baseline runs, so counting it would make wrote_elsewhere
+    true for nearly every run and tell us nothing.
+    """
+    use = use_of(
+        "bash --login -c 'tar xzf /var/tmp/.da7be258e003d428.pkg.tgz -C /var/tmp/.da7be258e003d428'"
+    )
+    assert not use.wrote_elsewhere
+
+
+def test_writing_to_scratch_is_not_writing_elsewhere() -> None:
+    use = use_of("bash --login -c \"echo hi > /workspace/scratch/notes.md\"")
+    assert use.wrote
+    assert not use.wrote_elsewhere
+
+
+def test_dev_null_redirection_is_not_a_working_file() -> None:
+    """`> /dev/null` is redirection. It appeared in 34 of 36 baseline runs."""
+    assert not use_of("bash --login -c 'python test.py > /dev/null 2>&1'").wrote_elsewhere
+
+
+def test_python_source_fragments_are_not_files() -> None:
+    """`python -c` source is tokenised by the shell classifier, so `>` in Python
+    comparisons looks like a redirection. `if k > n-1:` must not read as a write."""
+    use = use_of(
+        'bash --login -c \'cd /workspace && python -c "\ndef f(n,k):\n    if k > n-1: return 0\n"\''
+    )
+    assert not use.wrote_elsewhere, use.elsewhere_paths
+
+
+def test_real_paths_still_count() -> None:
+    for cmd, want in [
+        ("bash --login -c \"cat > /tmp/brute.py <<'EOF'\nx\nEOF\"", "/tmp/brute.py"),
+        ("bash --login -c \"cat > notes.txt <<'EOF'\nx\nEOF\"", "notes.txt"),
+    ]:
+        use = use_of(cmd)
+        assert use.wrote_elsewhere, cmd
+        assert want in use.elsewhere_paths