fix(inbox-send): write handoffs atomically so a failure leaves no phantom

inbox-send wrote straight to the destination path, and write_text truncates on open, so any mid-write failure left a zero-byte .org in another project's inbox. inbox-status counted that phantom as a pending handoff and blocked a turn in the receiving project over a file with no content and no sender.

Both send paths now write to a temp sibling and os.replace it into place, so the inbox only ever sees a complete file. A caught failure removes the temp and re-raises, leaving no debris. encoding is pinned to utf-8 on the write and on the roots-config read, which closes the locale-dependent failure that first surfaced this.

inbox-status also skips the .inbox-send-* temp, so the brief window before the rename can't read as pending either. The atomicity is only complete once the consumer ignores the in-flight file.

1 files changed, 72 insertions, 0 deletions

diff --git a/.ai/scripts/tests/test_inbox_send.py b/.ai/scripts/tests/test_inbox_send.py
index f75d7a1..02af23c 100644
--- a/.ai/scripts/tests/test_inbox_send.py
+++ b/.ai/scripts/tests/test_inbox_send.py
@@ -476,3 +476,75 @@ class TestFilenameCollisions:
         assert len(files) == 2
         bodies = "".join(f.read_text() for f in files)
         assert "message one" in bodies and "message two" in bodies
+
+
+class TestAtomicWrite:
+    """A send wrote straight to the destination path in another project's
+    inbox/, and write_text truncates on open, so any mid-write failure left a
+    zero-byte .org there. inbox-status counts that phantom as a pending
+    handoff, blocking a turn in the receiving project over a file with no
+    content (2026-07-23). The write must be atomic: the inbox sees a complete
+    file or nothing."""
+
+    def test_send_text_writes_utf8(self, tmp_path):
+        from datetime import datetime
+        mod = _load_module()
+        inbox = tmp_path / "inbox"
+        inbox.mkdir()
+        now = datetime(2026, 7, 23, 4, 36, 0)
+        # An em dash and an accented char — both non-ASCII.
+        dest = mod.send_text(inbox, "accent café and dash — here", "src", None, now)
+        # Reading as utf-8 must round-trip; a locale-encoded write would raise
+        # under a C locale, and reading back proves the bytes are utf-8.
+        assert "—" in dest.read_text(encoding="utf-8")
+
+    def test_send_text_no_partial_on_write_failure(self, tmp_path, monkeypatch):
+        from datetime import datetime
+        mod = _load_module()
+        inbox = tmp_path / "inbox"
+        inbox.mkdir()
+        now = datetime(2026, 7, 23, 4, 36, 0)
+        # Force the atomic finalize to fail after the temp file is written.
+        def boom(*a, **k):
+            raise OSError("disk full")
+        monkeypatch.setattr(mod.os, "replace", boom)
+        with pytest.raises(OSError):
+            mod.send_text(inbox, "a message that should never half-land", "src", None, now)
+        # No phantom, no leftover temp: the inbox is empty.
+        assert list(inbox.iterdir()) == []
+
+    def test_send_text_leaves_no_temp_on_success(self, tmp_path):
+        from datetime import datetime
+        mod = _load_module()
+        inbox = tmp_path / "inbox"
+        inbox.mkdir()
+        now = datetime(2026, 7, 23, 4, 36, 0)
+        dest = mod.send_text(inbox, "clean send", "src", None, now)
+        assert list(inbox.iterdir()) == [dest]
+
+    def test_send_file_no_partial_on_write_failure(self, tmp_path, monkeypatch):
+        from datetime import datetime
+        mod = _load_module()
+        inbox = tmp_path / "inbox"
+        inbox.mkdir()
+        src = tmp_path / "note.org"
+        src.write_text("body")
+        now = datetime(2026, 7, 23, 4, 36, 0)
+        def boom(*a, **k):
+            raise OSError("disk full")
+        monkeypatch.setattr(mod.os, "replace", boom)
+        with pytest.raises(OSError):
+            mod.send_file(inbox, src, "src", None, now)
+        assert list(inbox.iterdir()) == []
+
+    def test_send_file_leaves_no_temp_on_success(self, tmp_path):
+        from datetime import datetime
+        mod = _load_module()
+        inbox = tmp_path / "inbox"
+        inbox.mkdir()
+        src = tmp_path / "note.org"
+        src.write_text("payload")
+        now = datetime(2026, 7, 23, 4, 36, 0)
+        dest = mod.send_file(inbox, src, "src", None, now)
+        assert list(inbox.iterdir()) == [dest]
+        assert dest.read_text() == "payload"