Handle leading mentions in Matrix messages

Clients put the addressee's name in front of a message that mentions
it ("Nisse Dev Testing: re-plan"), and the transport passed that text
on whole, so no command matched: the live run's re-plan was asked again,
and an addressed 'approve' would have counted as rejection feedback.

- A leading mention of the agent (an HTML pill, its user ID, or its
  display name followed by ':') is removed before commands are matched.
  The display name is learnt from the agent's own member event.
- A message whose leading mention is someone else is skipped: in a
  shared room it is addressed to another agent or person.
- Only the start counts; a mention later in the text is left alone.
- The transcript records the message as sent (InboundMessage.original).

The sender is unaffected: it still comes from the event's homeserver-set
sender field, never from the text.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

nisse-dev/tests/test_matrix.py

@@ -143,3 +143,97 @@ def test_start_without_handler_raises() -> None:
     )
     with pytest.raises(TransportError, match="no message handler"):
         transport.start()
+
+
+# --- leading mentions --------------------------------------------------------
+
+
+def own_member_state(name: str) -> dict:
+    """The first sync's room state, which carries the agent's display name."""
+    body = sync_body([], "b1")
+    body["rooms"]["join"]["!room:test"]["state"] = {
+        "events": [
+            {
+                "type": "m.room.member",
+                "state_key": "@nisse-dev:test",
+                "sender": "@nisse-dev:test",
+                "content": {"membership": "join", "displayname": name},
+            }
+        ]
+    }
+    return body
+
+
+def pill_event(sender: str, user_id: str, label: str, rest: str) -> dict:
+    """A message as Element sends one that starts by mentioning user_id."""
+    return {
+        "type": "m.room.message",
+        "sender": sender,
+        "content": {
+            "msgtype": "m.text",
+            "body": f"{label}: {rest}",
+            "format": "org.matrix.custom.html",
+            "formatted_body": (
+                f'<a href="https://matrix.to/#/{user_id}">{label}</a>: {rest}'
+            ),
+        },
+    }
+
+
+def test_a_leading_pill_for_the_agent_is_removed_before_matching() -> None:
+    responses = [
+        sync_body([], "b1"),
+        sync_body(
+            [pill_event("@don:test", "@nisse-dev:test", "Nisse Dev", "approve")],
+            "b2",
+        ),
+    ]
+    [msg] = collect_messages(responses, expected=1)
+    assert msg.sender == "@don:test"  # from the event, never from the text
+    assert msg.text == "approve"
+    assert msg.original == "Nisse Dev: approve"
+
+
+def test_the_agents_display_name_typed_in_front_is_removed() -> None:
+    responses = [
+        own_member_state("Nisse Dev"),
+        sync_body([message_event("@don:test", "Nisse Dev: re-plan")], "b2"),
+    ]
+    [msg] = collect_messages(responses, expected=1)
+    assert msg.text == "re-plan"
+
+
+def test_the_agents_user_id_typed_in_front_is_removed() -> None:
+    responses = [
+        sync_body([], "b1"),
+        sync_body([message_event("@don:test", "@nisse-dev:test: status")], "b2"),
+    ]
+    [msg] = collect_messages(responses, expected=1)
+    assert msg.text == "status"
+
+
+def test_messages_addressed_to_someone_else_are_skipped() -> None:
+    responses = [
+        sync_body([], "b1"),
+        sync_body(
+            [
+                pill_event("@don:test", "@nisse-ops:test", "Nisse Ops", "approve"),
+                message_event("@don:test", "@alice:test: looks good"),
+                message_event("@don:test", "status"),
+            ],
+            "b2",
+        ),
+    ]
+    received = collect_messages(responses, expected=1)
+    assert received == [InboundMessage(sender="@don:test", text="status")]
+
+
+def test_a_mention_later_in_the_message_is_left_alone() -> None:
+    responses = [
+        sync_body([], "b1"),
+        sync_body([message_event("@don:test", "approve, cc @alice:test")], "b2"),
+    ]
+    received = collect_messages(responses, expected=1)
+    assert received == [
+        InboundMessage(sender="@don:test", text="approve, cc @alice:test")
+    ]