feat: initial m3ta-home structure

- flake.nix with nixpkgs, home-manager, nix-colors, m3ta-nixpkgs, agenix, NUR inputs
- lib/mkHome.nix: compose HM config from user + identity + context + sets
- profiles/base: shell, cli-tools, secrets (always loaded)
- profiles/contexts/desktop: WM, apps, theme, ghostty
- profiles/contexts/server: minimal headless
- profiles/sets/coding: core (git, direnv, jq, rg), editor, lsp, languages, agents
- profiles/sets/gaming: steam, gamescope, gpu
- profiles/sets/media: obs, ffmpeg, kdenlive, handbrake, yt-dlp
- users/m3tam3re/identities: private.nix, work.nix (git, jj, ssh per identity)
- users/m3tam3re/preferences: cliphist, difftastic, base packages

profiles/contexts/desktop/apps/obsidian.nix

@@ -0,0 +1,25 @@
+# Obsidian knowledge base with markdown MIME association.
+{
+  config,
+  lib,
+  ...
+}:
+with lib; let
+  cfg = config.desktop.apps.obsidian;
+in {
+  options.desktop.apps.obsidian.enable = mkEnableOption "enable Obsidian knowledge base";
+
+  config = mkIf cfg.enable {
+    programs.obsidian.enable = true;
+
+    xdg.mimeApps = {
+      enable = true;
+      associations.added = {
+        "text/markdown" = ["obsidian.desktop"];
+      };
+      defaultApplications = {
+        "text/markdown" = ["obsidian.desktop"];
+      };
+    };
+  };
+}