docs(test): centralize test infrastructure and add interactive agent harness

Added docs/TEST_INVENTORY.md as a central index for all tests and runners.
Created scripts/interactive_agent_tui_test.py for password-free TUI v2
exploratory testing. Integrated standalone permission tests into main
suite and fixed TUI v2 parity regression tests broken by auto-selection
removal. Updated AGENTS.md and CLAUDE.md with strategic plan references.

src/tests/test_atomic_write.py

@@ -75,3 +75,19 @@ def test_atomic_write_preserves_existing_on_failure(tmp_path: Path) -> None:
     # Should revert to 1 file
     assert len(list(tmp_path.iterdir())) == 1
     assert dest_path.read_text() == "original content"
+
+
+def test_atomic_write_permissions(tmp_path: Path) -> None:
+    """File permissions should be set to 0o600 (owner read/write only)."""
+    import stat
+
+    test_file = tmp_path / "permissions.txt"
+
+    def write_op(f):
+        f.write("content")
+
+    atomic_write(test_file, write_op)
+
+    st = test_file.stat()
+    mode = stat.S_IMODE(st.st_mode)
+    assert mode == 0o600