chore: clarify regenerate lifecycle warning wording (T90.2)

The warning said "lifecycle transitions from superseded turn ARE NOT
being rolled back". When regenerating an OLDER turn, the listed
transitions can include intervening-turn ones that legitimately stand
on their own — they weren't authored by the superseded turn itself.

Reword to "lifecycle transitions at-or-after turn <id>" so operators
reading logs aren't misled into thinking every listed event id was
emitted by the target turn. Cosmetic change to a single log message.

Test: extends test_regenerate_with_prior_lifecycle_logs_warning to
assert the new phrasing is present and the old phrasing is gone.

tests/test_regenerate.py

@@ -757,6 +757,13 @@ def test_regenerate_with_prior_lifecycle_logs_warning(tmp_path, monkeypatch, cap
     # row's id.
     assert str(at_id) in msg
     assert str(completed_id) in msg
+    # T90.2: wording was tightened from "from superseded turn" to
+    # "at-or-after turn <id>" — when regenerating an OLDER turn, the
+    # listed transitions may include legitimate intervening-turn ones
+    # that stand on their own. The new phrasing avoids implying the
+    # warning's target turn directly authored every listed transition.
+    assert "at-or-after turn" in msg
+    assert "from superseded turn" not in msg
 
 
 def test_regenerate_sibling_lookup_scoped_to_chat(tmp_path, monkeypatch):