fix(background-agent): detach error listener before running body to stop re-entrant log loop

When shutdown() itself emitted uncaughtException (e.g. EPIPE while closing
a broken pipe), the error listener re-entered itself, re-logged, re-ran
cleanup, and threw EPIPE again. The 6 s forced-exit timer could not fire
because every re-entry stalled the event loop with fresh synchronous work.

Users hit this after v3.17.5 and observed 100+ GB of log lines written to
disk within minutes, with one confirmed report of a 157 GB log file filling
the filesystem.

Detaching the listener with process.off() before running log() + handler()
breaks the loop at the first re-emit: the second event has no listener to
invoke, and the first invocation's scheduleForcedExit() proceeds normally.

src/features/background-agent/process-cleanup.ts

@@ -31,6 +31,12 @@ function registerErrorEvent(
   handler: (error: unknown) => void | Promise<void>
 ): (error: unknown) => void {
   const listener = (error: unknown) => {
+    // Detach before running the body so a re-emit from inside log()/handler()
+    // (e.g. EPIPE while closing a broken pipe during shutdown) cannot recurse.
+    // Prior behavior: the listener re-entered itself, re-logged, re-ran cleanup,
+    // and threw EPIPE again — an unbounded loop that filled disks with 100+ GB
+    // of log lines in minutes before the 6 s forced-exit timer could fire.
+    process.off(signal, listener)
     log(`[background-agent] ${signal} received during shutdown cleanup:`, error)
     scheduleForcedExit(handler(error), 1)
   }