feat(signal): make signal-cli attachments directory configurable

The inbound attachment loop hardcoded ~/.local/share/signal-cli/attachments
as the source path. That is the daemon's default on Linux but not on macOS
or Windows, and breaks if the daemon was launched with XDG_DATA_HOME set.

Add SignalConfig.attachments_dir as an optional override. When unset the
behavior is unchanged; when set the value is run through Path.expanduser()
so ~ is honored.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/channels/test_signal_channel.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import asyncio
+from pathlib import Path
 from unittest.mock import AsyncMock
 
 import pytest
@@ -71,6 +72,7 @@ def _make_channel(
     group_allow_from: list[str] | None = None,
     require_mention: bool = True,
     group_buffer_size: int = 20,
+    attachments_dir: str | None = None,
 ) -> SignalChannel:
     config = SignalConfig(
         enabled=True,
@@ -87,6 +89,7 @@ def _make_channel(
             require_mention=require_mention,
         ),
         group_message_buffer_size=group_buffer_size,
+        attachments_dir=attachments_dir,
     )
     return SignalChannel(config, MessageBus())
 
@@ -471,6 +474,21 @@ class TestGroupBuffer:
 # ---------------------------------------------------------------------------
 
 
+class TestAttachmentsDir:
+    def test_default_attachments_dir(self):
+        ch = _make_channel()
+        expected = Path.home() / ".local/share/signal-cli/attachments"
+        assert ch._signal_attachments_dir() == expected
+
+    def test_configured_attachments_dir(self, tmp_path):
+        ch = _make_channel(attachments_dir=str(tmp_path / "custom"))
+        assert ch._signal_attachments_dir() == tmp_path / "custom"
+
+    def test_attachments_dir_expands_user(self):
+        ch = _make_channel(attachments_dir="~/signal-attachments")
+        assert ch._signal_attachments_dir() == Path.home() / "signal-attachments"
+
+
 class TestHandleDataMessageDM:
     def _make_dm_channel(self, policy="open", allow_from=None) -> tuple[SignalChannel, list]:
         ch = _make_channel(dm_enabled=True, dm_policy=policy, dm_allow_from=allow_from or [])