feat(dank): enable DMS/niri session on mibook, pin HM to release-26.05, drop redundant keybinds

- Pin home-manager input to release-26.05 to match nixpkgs (was resolving
  to HM master / 26.11, causing a Home Manager version mismatch warning).
- Drop niri.enableKeybinds = true from the dank HM config; keybinds are
  already provided by the imported DMS niri homeModule (includes), and using
  both together triggered the "enableKeybinds and includes.enable" warning.
- Enable my.profiles.dank on mibook (machines/mibook/environments.nix).
- Rebuild confirmed exit 0 with both target warnings resolved.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018Sv2QAunLcLo3tS1YsfLgN

flake.nix

@@ -12,7 +12,7 @@
       inputs.nixpkgs.follows = "nixpkgs";
     };
     home-manager = {
-      url = "github:nix-community/home-manager";
+      url = "github:nix-community/home-manager/release-26.05";
       inputs.nixpkgs.follows = "nixpkgs";
     };
     niri.url = "github:sodiboo/niri-flake";