feat: add notesmd-cli package and flake checks

- Add notesmd-cli package for Obsidian CLI interaction
- Add flake checks for package build verification
- Add documentation for notesmd-cli, sidecar, and td
- Update AGENTS.md to reference td instead of beads
- Format pkgs/default.nix with proper braces

flake.nix

@@ -87,6 +87,19 @@
     # Formatter for 'nix fmt'
     formatter = forAllSystems (system: (pkgsFor system).alejandra);
 
+    # Checks for 'nix flake check' - verifies all packages build
+    checks = forAllSystems (system: let
+      pkgs = pkgsFor system;
+      packages = import ./pkgs {inherit pkgs inputs;};
+    in
+      builtins.mapAttrs (name: pkg: pkgs.lib.hydraJob pkg) packages
+      // {
+        formatting = pkgs.runCommand "check-formatting" {} ''
+          ${pkgs.alejandra}/bin/alejandra --check ${./.}
+          touch $out
+        '';
+      });
+
     # Templates for creating new packages/modules
     templates = {
       package = {