fix(agent): keep the tripping correction and explain the restart-bound exit

When the redirect cap trips, the correction that cancelled the final attempt is
still sitting in _pending_redirect; finalize_turn's clear_interrupt() would drop
it silently. Drain it into the steer slot so it rides result["pending_steer"] and
becomes the next user turn on every surface that already honours that key.

Both new exit reasons get a turn-completion explanation so the user sees why the
turn stopped instead of an empty reply.

agent/turn_explainers.py

@@ -41,6 +41,16 @@ _EXIT_REASON_EXPLANATIONS: Dict[str, str] = {
         "the request was interrupted mid-call before a reply was "
         "received. Send `continue` to retry."
     ),
+    "redirect_restart_limit_exceeded": (
+        "the request was cancelled by a new correction on every attempt, "
+        "so the turn stopped instead of retrying forever. Your last "
+        "correction is queued as the next message."
+    ),
+    "rebuilt_restart_limit_exceeded": (
+        "every provider in the fallback chain kept failing over, so the "
+        "turn stopped instead of retrying forever. Send `continue` or "
+        "switch provider."
+    ),
     "budget_exhausted": (
         "the per-turn iteration/cost budget was exhausted before a "
         "final answer. Send `continue` to keep going."