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.