fix(tools): prevent AbortSignal listener leak in exec and process tools

Each tool call added an abort listener to the shared agent signal
without cleanup, exceeding the default 10-listener limit after 11+
exec calls. Fix by using { once: true } and removing the listener
on child process close (exec) to prevent accumulation.

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

src/agent/tools/process.ts

@@ -112,7 +112,7 @@ export function createProcessTool(defaultCwd?: string): AgentTool<typeof Process
               if (signal) {
                 signal.addEventListener("abort", () => {
                   child.kill("SIGTERM");
-                });
+                }, { once: true });
               }
 
               resolve({ success: true });