refactor(signal): hygiene cleanups around constants, typing, and config

- Hoist the cell-strip patterns to module level so they match the rest of
  the module's regex style and aren't reparsed on every call.
- Type the markdown transform callback and the mention id walker so the
  inline Callable signature is no longer an untyped Any.
- Add _HTTP_TIMEOUT_SECONDS alongside the other class-level tunables.
- Reject group_message_buffer_size <= 0 in a Pydantic field_validator
  rather than silently disabling the buffer at write time.
- Mark SignalConfig.allow_from as a computed_field so it shows up in
  model_dump() instead of being invisible to serialization.

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

tests/channels/test_signal_channel.py

@@ -448,10 +448,13 @@ class TestGroupBuffer:
             ch._add_to_group_buffer("g1", "Alice", "+1111", f"msg{i}", i)
         assert len(ch._group_buffers["g1"]) == 3
 
-    def test_zero_buffer_size_does_not_add(self):
-        ch = _make_channel(group_buffer_size=0)
-        ch._add_to_group_buffer("g1", "Alice", "+1111", "msg", 1000)
-        assert "g1" not in ch._group_buffers
+    def test_zero_buffer_size_rejected_by_validator(self):
+        with pytest.raises(ValueError, match="group_message_buffer_size"):
+            _make_channel(group_buffer_size=0)
+
+    def test_negative_buffer_size_rejected_by_validator(self):
+        with pytest.raises(ValueError, match="group_message_buffer_size"):
+            _make_channel(group_buffer_size=-1)
 
     def test_context_limits_message_length(self):
         ch = _make_channel(group_buffer_size=5)