feat: add stop execution with hybrid graceful/force abort

Implement a stop execution feature that allows users to abort ongoing LLM
streaming, kill running tool calls, and clear pending permission/human input
requests. Uses a hybrid approach: first click sends graceful SIGTERM, second
click within 2s sends SIGKILL and force-closes MCP clients.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/x/packages/shared/src/ipc.ts

@@ -151,6 +151,7 @@ const ipcSchemas = {
   'runs:stop': {
     req: z.object({
       runId: z.string(),
+      force: z.boolean().optional().default(false),
     }),
     res: z.object({
       success: z.literal(true),