Sessions: update new chat isolation picker behavior (#300791)

* Sessions: reorder pickers, rename isolation labels, gate on config

- Swap mode and model picker positions in toolbar
- Move Default Approvals next to isolation mode picker with gap
- Rename Folder/Worktree labels to Local/Copilot CLI
- Gate isolation picker on github.copilot.chat.cli.isolationOption.enabled
- Show picker as disabled (not hidden) when config is off
- Add setEnabled API to IsolationModePicker
- Listen for config changes and enforce worktree mode when disabled

Co-authored-by: Copilot <[email protected]>

* revert

* Update src/vs/sessions/contrib/chat/browser/newChatViewPane.ts

Co-authored-by: Copilot <[email protected]>

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Co-authored-by: Copilot <[email protected]>
Co-authored-by: Copilot <[email protected]>