fix(send_message): accept Matrix room IDs and user MXIDs as explicit targets

`_parse_target_ref` has explicit-reference branches for Telegram, Feishu,
and numeric IDs, but none for Matrix. As a result, callers of
`send_message(target="matrix:!roomid:server")` or
`send_message(target="matrix:@user:server")` fall through to
`(None, None, False)` and the tool errors out with a resolution failure —
even though a raw Matrix room ID or MXID is the most unambiguous possible
target.

Three-line fix: recognize `!…` as a room ID and `@…` as a user MXID when
platform is `matrix`, and return them as explicit targets. Alias-based
targets (`#…`) continue to go through the normal resolve path.

tools/send_message_tool.py

@@ -248,6 +248,9 @@ def _parse_target_ref(platform_name: str, target_ref: str):
             return match.group(1), None, True
     if target_ref.lstrip("-").isdigit():
         return target_ref, None, True
+    # Matrix room IDs (start with !) and user IDs (start with @) are explicit
+    if platform_name == "matrix" and (target_ref.startswith("!") or target_ref.startswith("@")):
+        return target_ref, None, True
     return None, None, False