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/default.nix

@@ -0,0 +1,10 @@
+# Desktop apps aggregator — Obsidian, Office, web apps, crypto tools, and productivity.
+{...}: {
+  imports = [
+    ./obsidian.nix
+    ./office.nix
+    ./webapps.nix
+    ./crypto.nix
+    ./productivity.nix
+  ];
+}