Prevent Proliferation of signal.Notify handlers

Summary

Toolhive depends on signal.Notify in numerous places. In isolation, signal.Notify is not a problem, but relying on it in many places can create confusing and non-deterministic shutdown behavior.

Outside of main.go files, we should instead rely on clearer patterns for shutdown (e.g. context cancellation or ideally, callers invoking shutdown functions in deterministic order). Even with such patterns in place, we should discourage additional use of signal.Notify via a lint rule, so that all toolhive systems have easily maintainable shutdown behavior.

Definition of Done

A lint rule prevents use of signal.Notify or signal.NotifyContext outside of main.go files.

Existing callers outside of main.go are either removed or grandfathered in as ignored.

Additional Context

https://github.com/stacklok/toolhive/pull/2927#pullrequestreview-3553421250

Search the repository for signal.Notify and signal.NotifyContext, then identify the lint configuration and existing uses outside main.go. Review the shutdown patterns and the referenced pull request for context. Done means the lint rule rejects new uses outside main.go, while existing callers are removed or explicitly ignored.