feat: add frext text editor, replace sublimetext

[fredclausen]

Summary

Adds the frext text editor to the desktop config, following the
freminal pattern (flake input + home-manager module), and removes the
old sublimetext feature.

Changes

• flake.nix: add frext input (# CI: desktop) and wire it into
  sharedArgs; lock it in flake.lock.
• features/desktop/frext/: new feature module using the
  programs.frext home-manager module (not pkgs.frext, since frext is
  not in any overlay). Keeps the mimeApps default-application wiring for
  text/plain / application/x-zerosize, guarded to Linux only.
• features/desktop/default.nix: import ./frext, enable
  frext.enable = true, drop sublimetext.
• flake/hosts/nixos.nix + flake/hosts/darwin.nix: register
  frext.homeManagerModules.default on the desktop + darwin hosts,
  mirroring freminal.
• profiles/darwin.nix: import + enable frext.
• CI sync (four-place invariant): add frext to the
  input_category arrays in ci-linux.yaml (desktop),
  ci-darwin.yaml (darwin), and impacted-hosts.sh (desktop).

Verification

• nixfmt / statix / deadnix clean (pre-commit passed).
• nix eval of system.build.toplevel and HM activationPackage for
  Daytona and maranello: green, no evaluation warning:.
• Darwin configs cannot be evaluated locally on Linux; CI will validate.