fix: exempt the Matrix quote block from the mention strip only on real replies

The reply-pill fix split the body on a leading "> " so the strip would not
rewrite the "> <@bot:srv>" pill. That keyed the exemption on the body shape,
not on the relation: a hand-typed blockquote in a plain (non-reply) message
that mentions the bot inside the quote reached the agent with the raw
"@hermes:example.org" text, which main used to strip. Split around the quote
only when m.relates_to carries m.in_reply_to; otherwise strip the whole body.

Review finding: quote-block exemption keyed on body.startswith('> ') instead of m.in_reply_to.

plugins/platforms/matrix/adapter.py

@@ -2025,9 +2025,13 @@ class MatrixAdapter(BasePlatformAdapter):
             # Strip the mention from the reply text only: the quote block carries the
             # ``> <@bot:srv> ...`` reply pill, which _extract_reply_context parses later
             # for reply_to_author_id. A whole-body replace rewrote the pill to ``> <>``
-            # and silently dropped the replied-to author (#111233).
-            quote_block, reply_text = _split_reply_fallback(body)
-            body = quote_block + self._strip_mention(reply_text)
+            # and silently dropped the replied-to author (#111233). Only a real reply carries a
+            # pill; a hand-typed blockquote in a plain message is stripped whole as before.
+            if relates_to.get("m.in_reply_to"):
+                quote_block, reply_text = _split_reply_fallback(body)
+                body = quote_block + self._strip_mention(reply_text)
+            else:
+                body = self._strip_mention(body)
         # Real thread roots are preserved above; synthetic roots (this event) follow policy: DM
         # @mention threads / DM auto-thread, or room auto-thread unless session_scope pins the room.
         if not thread_id:

tests/gateway/test_matrix_message_event_metadata.py

@@ -290,3 +290,19 @@ async def test_reply_with_explicit_mention_still_strips_it_from_reply_text(monke
     assert msg.reply_to_author_id == "@carol:example.org"
     assert msg.reply_to_text == "original question"
     assert msg.text == "because reasons"
+
+
+@pytest.mark.asyncio
+async def test_plain_blockquote_without_reply_is_stripped_whole(monkeypatch):
+    """The quote exemption keys on m.in_reply_to, not on a leading ``> ``: a hand-typed
+    blockquote mentioning the bot in a non-reply message must not reach the agent raw."""
+    adapter = _make_adapter(require_mention=True, monkeypatch=monkeypatch)
+    adapter._startup_ts = time.time() - 10
+
+    event = _make_event("> @hermes:example.org please summarise\n\nthanks", event_id="$evt_quote")
+    await adapter._on_room_message(event)
+
+    msg = adapter.handle_message.await_args.args[0]
+    assert msg.reply_to_message_id is None
+    assert "@hermes:example.org" not in msg.text
+    assert "thanks" in msg.text