runtime/mcp: spawn servers in their own process group

A terminal Ctrl+C is delivered to every process in the foreground process
group, so a server spawned in our group died on any Ctrl+C during a turn -
killing in-flight tool calls and leaving the connection dead until
/mcp reconnect. start_new_session (POSIX) / CREATE_NEW_PROCESS_GROUP
(Windows) puts each server in its own session/group instead.

Tests assert the spawned server's process group and session differ from the
CLI's (they used to be the same group).

src/plyngent/runtime/mcp_client.py

@@ -11,11 +11,15 @@ import asyncio
 import importlib.metadata
 import json
 import os
+import subprocess
+import sys
 from collections import deque
 from dataclasses import dataclass
 from typing import TYPE_CHECKING, cast
 
 if TYPE_CHECKING:
+    from typing import Any
+
     from plyngent.config.models import McpConfig, McpServerConfig
 
 MCP_PROTOCOL_VERSION = "2025-06-18"
@@ -45,6 +49,19 @@ def _client_version() -> str:
         return "0.0.0"
 
 
+def _isolated_spawn_kwargs() -> dict[str, Any]:
+    """Spawn kwargs that keep a server out of our process/console group.
+
+    Ctrl+C in the terminal is delivered to every process in the foreground
+    process group (POSIX) / console process group (Windows), so without this a
+    single Ctrl+C during a turn also killed every MCP server — killing in-flight
+    tool calls and leaving the connection dead until ``/mcp reconnect``.
+    """
+    if sys.platform == "win32":
+        return {"creationflags": subprocess.CREATE_NEW_PROCESS_GROUP}
+    return {"start_new_session": True}
+
+
 class McpConnectionError(RuntimeError):
     """Server process failed to spawn, handshake, or answer in time."""
 
@@ -143,6 +160,7 @@ class McpServerConnection:
                 stderr=asyncio.subprocess.PIPE,
                 env=os.environ | self._config.env,
                 cwd=self._config.cwd or None,
+                **_isolated_spawn_kwargs(),
             )
         except (OSError, ValueError) as exc:
             self.error = f"spawn {self._config.command!r}: {exc}"