fix: route deferred update notice through cprint to avoid ANSI garbling

The deferred update notice ("N commits behind") was rendered via
console.print which writes ANSI escapes directly to stdout. Under
patch_stdout (active in the interactive CLI), the StdoutProxy strips
ESC bytes, leaving visible [1;33m...[0m artifacts.

Fix: render Rich markup to an ANSI string via a captured Console, then
route through cprint (which uses prompt_toolkit ANSI parser) to
bypass the StdoutProxy mangling.

This is the same class of bug as #2262 (fixed by #2448 for agent._print_fn
and display.py), but the _defer_update_notice callsite in banner.py was
not covered by that fix.

Fixes #83969

hermes_cli/banner.py

@@ -491,11 +491,25 @@ def _format_update_notice(behind: int) -> str:
 _deferred_update_notice_started = False
 
 
+def _render_markup_to_ansi(markup: str) -> str:
+    """Rich markup → ANSI string, for output that must go through prompt_toolkit's renderer.
+
+    Under ``patch_stdout`` (the interactive CLI), a plain ``Console.print`` writes ESC bytes into
+    the StdoutProxy, which sanitizes them into visible ``?[1;33m…`` artifacts (#83969).
+    """
+    from io import StringIO
+    from rich.console import Console as _Console
+    buf = StringIO()
+    _Console(file=buf, force_terminal=True, color_system="truecolor", highlight=False).print(markup)
+    return buf.getvalue().rstrip("\n")
+
+
 def _defer_update_notice(console: "Console", max_wait: float = 30.0) -> None:
     """Print the update warning once the prefetched check completes (at most once per process).
 
     Used when the banner rendered before the update prefetch finished so startup never blocks on
-    git/network.
+    git/network. The notice lands after prompt_toolkit owns the terminal, so it is routed through
+    ``cprint`` (prompt_toolkit's renderer prints above a running application from any thread).
     """
     global _deferred_update_notice_started
     if _deferred_update_notice_started:
@@ -504,7 +518,7 @@ def _defer_update_notice(console: "Console", max_wait: float = 30.0) -> None:
 
     def _wait_and_print() -> None:
         if _update_check_done.wait(timeout=max_wait) and _update_result:
-            console.print(_format_update_notice(_update_result))
+            cprint(_render_markup_to_ansi(_format_update_notice(_update_result)))
     _daemon("update-notice", _wait_and_print)  # never break the session over an update notice
 
 
@@ -863,8 +877,7 @@ def build_welcome_banner(
     right_lines.append(f"[dim {dim}]{' · '.join(summary_parts)}[/]")
     # Update check — NEVER block the banner on it: the prefetch does git/network work that rarely
     # finishes before render, so a blocking wait adds its full timeout to every startup. If not
-    # ready, a daemon thread prints the same notice above the prompt when it lands
-    # (prompt_toolkit's patch_stdout renders late prints safely).
+    # ready, a daemon thread prints the same notice above the prompt when it lands.
     def _update_line():
         behind = get_update_result(timeout=0.05)
         if behind is None and not _update_check_done.is_set():

tests/tools/test_startup_latency_regressions.py

@@ -175,37 +175,41 @@ class TestBannerUpdateCheckNonBlocking:
 
         printed = []
 
-        class _Console:
-            def print(self, msg, *a, **k):
-                printed.append(msg)
+        def _fake_cprint(text):
+            printed.append(text)
 
         done = threading.Event()
         with patch.object(banner, "_update_check_done", done), \
              patch.object(banner, "_update_result", None), \
-             patch.object(banner, "_deferred_update_notice_started", False):
-            banner._defer_update_notice(_Console(), max_wait=5.0)
+             patch.object(banner, "_deferred_update_notice_started", False), \
+             patch.object(banner, "cprint", _fake_cprint):
+            banner._defer_update_notice(None, max_wait=5.0)
             banner._update_result = 3
             done.set()
             deadline = time.time() + 5
             while not printed and time.time() < deadline:
                 time.sleep(0.02)
         assert printed, "deferred update notice never printed"
-        assert "3 commits behind" in printed[0]
+        # cprint receives ANSI-rendered text (ESC escapes + visible text),
+        # so check that the visible payload contains the expected message.
+        import re
+        visible = re.sub(r"\x1b\[[0-9;]*m", "", printed[0])
+        assert "3 commits behind" in visible
 
     def test_deferred_notice_silent_when_up_to_date(self):
         import hermes_cli.banner as banner
 
         printed = []
 
-        class _Console:
-            def print(self, msg, *a, **k):
-                printed.append(msg)
+        def _fake_cprint(text):
+            printed.append(text)
 
         done = threading.Event()
         with patch.object(banner, "_update_check_done", done), \
              patch.object(banner, "_update_result", 0), \
-             patch.object(banner, "_deferred_update_notice_started", False):
-            banner._defer_update_notice(_Console(), max_wait=2.0)
+             patch.object(banner, "_deferred_update_notice_started", False), \
+             patch.object(banner, "cprint", _fake_cprint):
+            banner._defer_update_notice(None, max_wait=2.0)
             done.set()
             time.sleep(0.3)
         assert not printed