fix: render markdown in assistant panels and speed up reasoning-only recovery

Assistant message panels now use Markdown() instead of raw strings, so
bold/italic/lists render properly. Also nudge the model immediately after
tool errors instead of wasting 2 retry iterations in reasoning-only mode.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/utils/display.py

@@ -10,6 +10,7 @@ from __future__ import annotations
 
 from typing import TYPE_CHECKING, Protocol
 
+from rich.markdown import Markdown
 from rich.panel import Panel
 from rich.table import Table
 from rich.text import Text
@@ -50,7 +51,7 @@ def render_user_message(content: str) -> Panel:
 
 def render_assistant_message(content: str) -> Panel:
     """Render an assistant message as a styled panel."""
-    return Panel(content, title="Assistant", border_style="green", expand=True)
+    return Panel(Markdown(content), title="Assistant", border_style="green", expand=True)
 
 
 def render_tool_call(name: str, args: str) -> Text: