fix: anchor picker menus to ButtonGroup, remove overflow hidden

[adamleithp]

Summary

• Add anchor prop to FolderPicker, GitHubRepoPicker, and BranchSelector so their dropdown/combobox menus can anchor to a parent element
• TaskInput passes a buttonGroupRef from the ButtonGroup wrapper to all three pickers, so menus align to the group without breaking its layout
• Remove overflow: hidden from TaskInput container that was clipping focus rings and potentially popover content

Test plan

• Open folder picker — dropdown anchors to the button group
• Open GitHub repo picker (cloud mode) — combobox anchors to the button group
• [ x Open branch selector — combobox anchors to the button group
• [ x Verify focus rings on inputs aren't clipped
• Verify BranchSelector still works standalone (e.g. in HeaderRow) without an anchor prop — falls back to its own internal ref

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