feat(tui): add markdown rendering for agent messages (#197)

Add AgentMessageRenderer to render agent messages with basic markdown support:
- Headers (#, ##, ###, ####)
- Bold (**text**) and italic (*text*)
- Inline code and fenced code blocks
- Links [text](url) and strikethrough

Update system prompt to allow agents to use simple markdown formatting.

strix/interface/tool_components/agent_message_renderer.py

@@ -0,0 +1,70 @@
+import re
+from typing import Any, ClassVar
+
+from textual.widgets import Static
+
+from .base_renderer import BaseToolRenderer
+from .registry import register_tool_renderer
+
+
+def markdown_to_rich(text: str) -> str:
+    # Fenced code blocks: ```lang\n...\n``` or ```\n...\n```
+    text = re.sub(
+        r"```(?:\w*)\n(.*?)```",
+        r"[dim]\1[/dim]",
+        text,
+        flags=re.DOTALL,
+    )
+
+    # Headers
+    text = re.sub(r"^#### (.+)$", r"[bold]\1[/bold]", text, flags=re.MULTILINE)
+    text = re.sub(r"^### (.+)$", r"[bold]\1[/bold]", text, flags=re.MULTILINE)
+    text = re.sub(r"^## (.+)$", r"[bold]\1[/bold]", text, flags=re.MULTILINE)
+    text = re.sub(r"^# (.+)$", r"[bold]\1[/bold]", text, flags=re.MULTILINE)
+
+    # Links
+    text = re.sub(r"\[([^\]]+)\]\(([^)]+)\)", r"[underline]\1[/underline] [dim](\2)[/dim]", text)
+
+    # Bold
+    text = re.sub(r"\*\*(.+?)\*\*", r"[bold]\1[/bold]", text)
+    text = re.sub(r"__(.+?)__", r"[bold]\1[/bold]", text)
+
+    # Italic
+    text = re.sub(r"(?<!\*)\*(?!\*)(.+?)(?<!\*)\*(?!\*)", r"[italic]\1[/italic]", text)
+    text = re.sub(r"(?<![_\w])_(?!_)(.+?)(?<!_)_(?![_\w])", r"[italic]\1[/italic]", text)
+
+    # Inline code
+    text = re.sub(r"`([^`]+)`", r"[bold dim]\1[/bold dim]", text)
+
+    # Strikethrough
+    return re.sub(r"~~(.+?)~~", r"[strike]\1[/strike]", text)
+
+
+@register_tool_renderer
+class AgentMessageRenderer(BaseToolRenderer):
+    tool_name: ClassVar[str] = "agent_message"
+    css_classes: ClassVar[list[str]] = ["chat-message", "agent-message"]
+
+    @classmethod
+    def render(cls, message_data: dict[str, Any]) -> Static:
+        content = message_data.get("content", "")
+
+        if not content:
+            return Static("", classes=cls.css_classes)
+
+        formatted_content = cls._format_agent_message(content)
+
+        css_classes = " ".join(cls.css_classes)
+        return Static(formatted_content, classes=css_classes)
+
+    @classmethod
+    def render_simple(cls, content: str) -> str:
+        if not content:
+            return ""
+
+        return cls._format_agent_message(content)
+
+    @classmethod
+    def _format_agent_message(cls, content: str) -> str:
+        escaped_content = cls.escape_markup(content)
+        return markdown_to_rich(escaped_content)