Add obsidian-skills as a flake input (flake = false) and map each
skill subdirectory into ~/.agents/skills/<skill>, alongside the
existing local skills. Updates flow through `nix flake update
obsidian-skills`.
Adds metacfg.cli-apps.opencode (default enabled) which mounts the
in-repo opencode config (provider list, web-search skill) via
xdg.configFile, so all hosts pick it up automatically.