Hide closed sub-issues [GA]

### Value Prop
When you're tracking a long-running project with many sub-issues, completed items can quickly overwhelm the list and make it hard to see what still needs attention. A new "Hide closed issues" toggle in the sub-issues pane lets you instantly clear completed work from view so you can focus on what's still open. You can bring closed items back at any time, keeping full visibility without the clutter.

### Expected Outcome
Teams using umbrella issues to track complex projects spend less time scrolling past finished work and more time acting on what's left to do. This improvement makes issue trackers easier to scan at a glance, reducing friction for anyone managing work across many tasks. The result is a faster, cleaner workflow for staying on top of active work in progress.

The issue names no repository files, tests, or entry points. Start by locating the sub-issues pane and its handling of open and closed items; done means adding a toggle that hides closed sub-issues while allowing them to be shown again.

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