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_control.py

@@ -4,6 +4,7 @@ from pathlib import Path
 
 import pytest
 
+from nisse_dev.canonical import UserMessage
 from nisse_dev.control import (
     AWAIT_ARCH_SIGNOFF,
     AWAIT_INPUT,
@@ -125,6 +126,19 @@ def test_start_milestone_requires_approval_first(h: Harness) -> None:
     assert h.state == EXECUTING
 
 
+def test_a_message_is_matched_as_stripped_but_recorded_as_sent(
+    h: Harness,
+) -> None:
+    h.transport.receive("don", "use demo")
+    h.transport.receive("don", "approve", original="Nisse Dev: approve")
+    assert h.state == AWAIT_MILESTONE_SIGNOFF
+    recorded = [
+        e.event.text for e in h.store.read_events() if isinstance(e.event, UserMessage)
+    ]
+    assert "Nisse Dev: approve" in recorded
+    assert "approve" not in recorded
+
+
 def test_status_reports_state_and_gate(h: Harness) -> None:
     h.transport.receive("don", "use demo")
     h.transport.receive("don", "status")