fix(config): restore read-only-target protection for atomic writes

Review finding 2 (10.07 gate): a deliberately locked config (0444) in a
writable directory was silently overwritten — atomic replace creates a
fresh temp inode and renames over the read-only file, defeating the old
in-place Path.write_text PermissionError contract.

Probe the existing target with a non-truncating O_WRONLY open before any
replacement work and let the PermissionError propagate. The probe fstat()s
the fd it actually opened and hands that stat to the rest of the write, so
a concurrent writer replacing the inode between stat and probe refreshes
the metadata instead of failing (keeps concurrent-writer semantics).

Regression: writable parent + 0444 target now raises and keeps the
original bytes and mode.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

api/paths.py

@@ -83,6 +83,28 @@ def _has_extended_attributes(path: Path) -> bool:
         raise
 
 
+def _require_writable_target(write_path: Path) -> os.stat_result | None:
+    """Reject writes to a read-only target before any replacement work.
+
+    ``os.replace`` happily swaps a fresh inode over a ``0444`` file when the
+    directory is writable, which would silently defeat a deliberately locked
+    config. The old in-place ``Path.write_text`` raised ``PermissionError``
+    there; preserve that contract with a non-truncating ``O_WRONLY`` probe,
+    letting the failure propagate. Returns the ``fstat`` of the inode the
+    probe actually opened (so the caller's metadata describes the verified
+    file even if a concurrent writer replaced it since its own ``stat``), or
+    ``None`` if the target vanished concurrently.
+    """
+    try:
+        probe_fd = os.open(write_path, os.O_WRONLY)
+    except FileNotFoundError:
+        return None
+    try:
+        return os.fstat(probe_fd)
+    finally:
+        os.close(probe_fd)
+
+
 def _atomic_write_text(path: Path, text: str, *, encoding: str = "utf-8") -> None:
     """Atomically replace *path* with *text*.
 
@@ -168,13 +190,19 @@ def _atomic_write_text(path: Path, text: str, *, encoding: str = "utf-8") -> Non
                 if owns_fallback_fd:
                     os.close(fallback_fd)
 
-    if (
-        existing_stat is not None
-        and stat.S_ISREG(existing_stat.st_mode)
-        and (existing_stat.st_nlink > 1 or _has_extended_attributes(write_path))
-    ):
-        _write_in_place()
-        return
+    if existing_stat is not None and stat.S_ISREG(existing_stat.st_mode):
+        probed_stat = _require_writable_target(write_path)
+        if probed_stat is not None and stat.S_ISREG(probed_stat.st_mode):
+            existing_stat = probed_stat
+            mode = stat.S_IMODE(existing_stat.st_mode)
+        else:
+            existing_stat = probed_stat
+            mode = None
+        if existing_stat is not None and (
+            existing_stat.st_nlink > 1 or _has_extended_attributes(write_path)
+        ):
+            _write_in_place()
+            return
 
     try:
         fd, tmp = _create_atomic_temp_file(write_path, existing=existing_stat is not None)

tests/test_5774b_atomic_config_writes.py

@@ -786,6 +786,26 @@ def test_readonly_parent_without_writable_target_still_raises(
 
 
 @_ROOT_SKIP
+def test_writable_parent_with_readonly_file_raises_and_keeps_bytes(tmp_path: Path) -> None:
+    """A deliberately locked config (0444) in a writable directory must keep
+    rejecting writes: without the target-writability probe, atomic replace
+    creates a fresh temp inode and renames over the read-only file."""
+    cfg_dir = tmp_path / "open-dir"
+    cfg_dir.mkdir()
+    target = cfg_dir / "config.yaml"
+    original = "model:\n  default: keep-me\n"
+    target.write_text(original, encoding="utf-8")
+    os.chmod(target, 0o444)
+    os.chmod(cfg_dir, 0o755)
+    try:
+        with pytest.raises(PermissionError):
+            _atomic_write_text(target, "model:\n  default: new\n")
+        assert target.read_text(encoding="utf-8") == original
+        assert stat.S_IMODE(os.stat(target).st_mode) == 0o444
+    finally:
+        os.chmod(target, 0o644)
+
+
 def test_readonly_parent_with_unwritable_file_still_raises(tmp_path: Path) -> None:
     cfg_dir = tmp_path / "locked"
     cfg_dir.mkdir()