Root cause: gateway spawns LSP servers (jdtls/pyright/yaml-ls) and
slash_worker without start_new_session=True, so they inherit the
gateway process group (= TUI parent PID). When mcp_tool
_snapshot_child_pids() races with these spawns during stdio MCP
server startup, non-MCP children leak into _stdio_pgids with the
TUI parent PGID. shutdown_mcp_servers() then killpg(tui_parent_pid,
SIGTERM), killing the TUI itself.
Evidence: tui_gateway_crash.log shows recurring SIGTERM stacks:
shutdown_mcp_servers -> _kill_orphaned_mcp_children ->
_send_signal -> killpg(pgid, sig) -> SIGTERM received
Fix (3 layers):
1. agent/lsp/client.py: add start_new_session=True to LSP server
spawn so each LSP server gets its own process group/session.
2. tui_gateway/server.py: same fix for slash_worker spawn, the
symmetric root-cause patch so no gateway direct child shares
the TUI parent pgid.
3. tools/mcp_tool.py: add _filter_mcp_children() defense-in-depth
that drops non-MCP children (slash_worker, jdtls/eclipse LSP)
from the PID delta before they can poison _stdio_pgids.
18a9467fca