fix(agent): resolve relative media paths in MessageTool

When deployed with Docker and workspace mounted as a volume, sending
media files failed because relative paths (e.g. output/image.png) were
not resolved against the workspace directory. The process CWD differs
from the workspace in containerized environments, causing os.path.isfile
checks to fail in channel handlers. Normalize relative media paths at
the MessageTool entry point using get_workspace_path().

nanobot/agent/tools/message.py

@@ -1,11 +1,13 @@
 """Message tool for sending messages to users."""
 
+import os
 from contextvars import ContextVar
 from typing import Any, Awaitable, Callable
 
 from nanobot.agent.tools.base import Tool, tool_parameters
 from nanobot.agent.tools.schema import ArraySchema, StringSchema, tool_parameters_schema
 from nanobot.bus.events import OutboundMessage
+from nanobot.config.paths import get_workspace_path
 
 
 @tool_parameters(
@@ -141,6 +143,15 @@ class MessageTool(Tool):
         if not self._send_callback:
             return "Error: Message sending not configured"
 
+        if media:
+            resolved = []
+            for p in media:
+                if p.startswith(("http://", "https://")) or os.path.isabs(p):
+                    resolved.append(p)
+                else:
+                    resolved.append(str(get_workspace_path() / p))
+            media = resolved
+
         metadata = dict(self._default_metadata.get()) if same_target else {}
         if message_id:
             metadata["message_id"] = message_id