[codex] Add namespace mkdir and attr hotkeys

[lobis]

Summary

• add m in the namespace view to open a new-directory popup
• resolve entered names relative to the current namespace path, while still allowing absolute paths
• run eos mkdir <path> and refresh the current directory on success
• keep enter for namespace attribute editing and add a as the same attr-edit hotkey
• reload namespace attrs after applying a namespace filter so enter/a targets the visible selected row
• add CI coverage for the filtered-row attr regression and the full enter-driven attr lifecycle
• advertise m mkdir and enter/a attrs in the namespace footer

Validation

• go test ./eos -run TestMkdirRunsEOSMkdir
• go test ./ui -run 'TestNamespace(FilterReloadsAttrsForVisibleSelection|EnterAttrsWorksAfterFiltering|EnterOpensAttributeEditor|AOpensAttributeEditor|EnterAttributeEditorLifecycle|MOpensNewDirectoryPopup|NDoesNotOpenNewDirectoryPopup|MkdirEnterRunsCommand|MkdirResultRefreshesDirectory|FooterAdvertisesMkdirHotkey)'\n- go test ./...\n- go build ./...\n