#cmux 0.64.9: 0.64.8 Memory Leak Hotfix

Fix 8GB RSS Growth on Non-Git Workspaces. Git repository search now stops at filesystem root instead of walking forever into `/..`. On older Foundation builds, `deletingLastPathComponent()` could yield `/..` and keep climbing, allocating ever-longer parent paths until the OS OOM killer fired. Non-Git workspaces no longer grow RSS from ~450MB to 8GB within minutes on 0.64.8.

Restore Browser Memory Saver Default. Hidden browser webview renderers discard by default again, reverting the 0.64.8 keep-alive default that exposed the memory regression. The keep-alive behavior remains available as an opt-in setting for workflows that need to preserve DOM state across workspace switches.

• Stop unbounded Git repository search past filesystem root so non-Git workspaces no longer grow RSS from ~450MB to 8GB and trigger the OOM killer (#4557) -- thanks @Luciferxie for the report!
• Restore the Browser Memory Saver default to on (discards hidden browser webview renderers after the discard delay) to mitigate the 0.64.8 memory regression (#4545) -- thanks @Luciferxie for the report!