fix: use signal.NotifyContext for container cleanup on SIGINT/SIGTERM

[bfirsh]

Summary

• Replace manual goroutine-based signal handlers in cog predict and cog train with signal.NotifyContext, ensuring containers are always cleaned up on SIGINT/SIGTERM
• Remove redundant signal-catching goroutines that used the parent context (which could already be cancelled)
• Remove FIXME: will not run on signal comments since the issue is now resolved

Problem

Both pkg/cli/predict.go and pkg/cli/train.go had two competing signal-handling mechanisms, neither fully correct:

1. A goroutine with signal.Notify that caught SIGINT and called predictor.Stop(ctx) — but used the parent context which could already be cancelled
2. A deferred predictor.Stop(context.Background()) — but Go defers don't run when the process exits from a signal handler

This meant hitting Ctrl+C during cog predict or cog train could leave orphaned Docker containers running.

Solution

Use signal.NotifyContext to create a context that is automatically cancelled on SIGINT/SIGTERM. When the signal arrives, in-flight operations (e.g. predictor.Start(), predictor.Predict()) return with a context cancellation error, the function returns normally, and the deferred predictor.Stop(context.Background()) runs to clean up the container.

Fixes #2799