Increase multiplexer fork buffer to 4096 to prevent output drops

64-entry buffer was too small for high-throughput PTY output (e.g.
ls -laihR /). The consumer couldn't drain fast enough over the RPC
stream, causing the non-blocking send fallback to silently discard
data. 4096 entries (~64MB at 16KB/chunk) handles sustained output
without drops while still preventing deadlock on stuck consumers.

envd/internal/services/process/handler/multiplex.go

@@ -56,7 +56,7 @@ func (m *MultiplexedChannel[T]) Fork() (chan T, func()) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
 
-	consumer := make(chan T, 64)
+	consumer := make(chan T, 4096)
 
 	m.channels = append(m.channels, consumer)