Add Discord-native hybrid front-end for Jarvis (bot + bridge)

Transform isair/jarvis into a Discord-controlled voice assistant running on
the Ubuntu VNC desktop, keeping the mature ~39k-line Python brain intact.

- bot/ (Node + bun, discord.js): /자비스 slash commands (ephemeral),
  voice channel join + voice receive/playback, pluggable VNC screen broadcast
  (selfbot live / noVNC / screenshot)
- bridge/ (Python, Flask): wraps jarvis STT + run_reply_engine + Piper TTS
  behind a thin localhost HTTP API
- .env.example, scripts/ (start_bridge/start_bot/dev), README rewrite,
  docs/language-comparison.md and docs/vnc-xfce-setup.md

Language decision: hybrid (Python brain + Node/bun Discord layer) because
Discord blocks bot video; native screen broadcast only works via a Node
selfbot library.

tests/tools/builtin/test_stop.py

@@ -0,0 +1,68 @@
+"""Tests for stop tool."""
+
+import pytest
+from unittest.mock import Mock
+
+from src.jarvis.tools.builtin.stop import StopTool, STOP_SIGNAL
+from src.jarvis.tools.base import ToolContext
+from src.jarvis.tools.types import ToolExecutionResult
+
+
+class TestStopTool:
+    """Test stop tool functionality."""
+
+    def setup_method(self):
+        """Set up test fixtures."""
+        self.tool = StopTool()
+        self.context = Mock(spec=ToolContext)
+        self.context.user_print = Mock()
+
+    def test_tool_properties(self):
+        """Test tool metadata properties."""
+        assert self.tool.name == "stop"
+        assert "end" in self.tool.description.lower()
+        assert "conversation" in self.tool.description.lower()
+        assert self.tool.inputSchema["type"] == "object"
+        assert self.tool.inputSchema["required"] == []
+        assert self.tool.inputSchema["properties"] == {}
+
+    def test_run_returns_stop_signal(self):
+        """Test that run returns the special stop signal."""
+        result = self.tool.run({}, self.context)
+
+        assert isinstance(result, ToolExecutionResult)
+        assert result.success is True
+        assert result.reply_text == STOP_SIGNAL
+        assert result.error_message is None
+
+    def test_run_with_none_args(self):
+        """Test that run works with None args."""
+        result = self.tool.run(None, self.context)
+
+        assert isinstance(result, ToolExecutionResult)
+        assert result.success is True
+        assert result.reply_text == STOP_SIGNAL
+
+    def test_stop_signal_is_unique(self):
+        """Test that stop signal is a unique value unlikely to be confused with real content."""
+        assert STOP_SIGNAL.startswith("__")
+        assert STOP_SIGNAL.endswith("__")
+        assert "JARVIS" in STOP_SIGNAL
+        assert "STOP" in STOP_SIGNAL
+
+
+class TestStopSignalIntegration:
+    """Test stop signal integration with registry."""
+
+    def test_stop_tool_in_registry(self):
+        """Test that stop tool is registered in BUILTIN_TOOLS."""
+        from src.jarvis.tools.registry import BUILTIN_TOOLS
+
+        assert "stop" in BUILTIN_TOOLS
+        assert isinstance(BUILTIN_TOOLS["stop"], StopTool)
+
+    def test_stop_tool_always_available(self):
+        """Test that stop tool is available to all profiles via BUILTIN_TOOLS."""
+        from src.jarvis.tools.registry import BUILTIN_TOOLS
+
+        assert "stop" in BUILTIN_TOOLS, "stop tool must be in BUILTIN_TOOLS"