Make the Tests workflow able to fail on Windows, and skip two POSIX-only tests

The Tests workflow reported SUCCESS on Windows while its own log read
"2 failed, 1114 passed". On Windows a multi-line `run:` block executes under
pwsh, where only the last command decides the step's result -- so
`coverage xml` succeeding hid `pytest` failing. Linux and macOS use bash -e
and did catch it, which is why this was invisible. `shell: bash` on that step
fixes it, and the same hazard is fixed in python-ci.yml's install step. An
audit of every Windows-capable multi-line step found no others; the installer
smoke steps are Unix-guarded and already set -euo pipefail.

A check that cannot go red is worse than no check, because it is trusted --
and this one has been trusted while six Windows tests failed underneath it.

Two of those six are now skipped on Windows: they assert POSIX mode 0o600,
which cannot hold where os.chmod only toggles a read-only flag and st_mode
reads back 0o666. Skipped rather than relaxed, because the property is real
and a weakened assertion would answer it falsely. Whether these files -- the
semantic index among them, which holds stored secrets -- are actually
protected from other users on Windows is an ACL question, now recorded.

The remaining four are TUI tests that assume POSIX paths. They are recorded
rather than patched: reproducing them needs a Windows host, and guessing at
path normalization and palette-argument parsing would mean changing tests to
match an assumption rather than a behaviour. They also interact with the
undeclared-textual finding -- those tests SKIP everywhere except Windows, the
one platform they were not written for.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01Vtx94nXDmNgXEeThUZP4QN

src/tests/test_atomic_write.py

@@ -1,6 +1,7 @@
 import json
 import pytest
 from multiprocessing import Process
+import sys
 from pathlib import Path
 
 from utils.atomic_write import atomic_write
@@ -77,6 +78,17 @@ def test_atomic_write_preserves_existing_on_failure(tmp_path: Path) -> None:
     assert dest_path.read_text() == "original content"
 
 
+@pytest.mark.skipif(
+    sys.platform == "win32",
+    reason=(
+        "POSIX mode bits do not exist on Windows: os.chmod only toggles the "
+        "read-only flag and st_mode reads back 0o666, so this assertion cannot "
+        "hold there. It is SKIPPED rather than relaxed because the property it "
+        "checks is real -- whether these files are protected from other users "
+        "on Windows is an open question about ACLs, recorded in TODO.md, and "
+        "a weakened assertion would answer it falsely."
+    ),
+)
 def test_atomic_write_permissions(tmp_path: Path) -> None:
     """File permissions should be set to 0o600 (owner read/write only)."""
     import stat