add obsidian home-manager profile with vim config and community plugins
Configure Obsidian declaratively via the new home-manager programs.obsidian
module. Packages 5 community plugins (linter, quickadd, recent-files,
periodic-notes, vimrc-support) and the Ayu Mirage theme as nix derivations
fetched from GitHub releases. Shared defaultSettings across kitaab/markdown
(all machines) and usr/acreom/sourcegraph (darwin-only) vaults.