Connect to Discord, switch to NVIDIA NIM providers

- Enable Discord channel, respond to all messages from allowed users
- Switch all agents to NVIDIA NIM (Kimi K2.5, DeepSeek V3.1)
- Auto-approve all tools for non-interactive Docker deployment
- Fix tool call arguments serialization (dict → JSON string)
- Fix Dockerfile to copy source before uv sync

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

xtrm_agent/engine.py

@@ -7,6 +7,8 @@ from typing import Any
 from loguru import logger
 
 from xtrm_agent.config import AgentFileConfig
+import json
+
 from xtrm_agent.llm.provider import LLMProvider, LLMResponse
 from xtrm_agent.tools.approval import ApprovalEngine
 from xtrm_agent.tools.registry import ToolRegistry
@@ -100,7 +102,7 @@ class Engine:
                 {
                     "id": tc.id,
                     "type": "function",
-                    "function": {"name": tc.name, "arguments": tc.arguments},
+                    "function": {"name": tc.name, "arguments": json.dumps(tc.arguments)},
                 }
                 for tc in response.tool_calls
             ]