Update gitlab-ci to 0.19.0

Summary

Updates gitlab-ci from 0.10.1 to 0.19.0 and the .gitlab-ci.yml template from 0.9.0 to 0.19.0. Removes the unused get-flake input and migrates to the new strict-inputs API.

Changes

gitlab-ci 0.10.1 → 0.19.0

• Updated flake.nix: gitlab-ci input now points to 0.19.0
• Updated flake.lock: all gitlab-ci entries updated

.gitlab-ci.yml template 0.9.0 → 0.19.0

• Updated .gitlab-ci.yml to reference the 0.19.0 CI template

Removed get-flake input

• get-flake is no longer used in the codebase
• Removed from flake.nix inputs and flake.lock
• Removed from nix/flake-parts-modules.nix exports

strict-inputs migration to new API

• Old: gitlab.ci.strict-flake-inputs.enable
• New: gitlab.ci.flake.strict-inputs.enable
• Updated nix/modules/flake/horizon-ci.nix accordingly
• Added exceptions for common upstream dependencies that don't use protected refs

nixpkgs ref compliance

• Changed nixpkgs inputs to use refs/tags/ prefix for strict-inputs compliance

Other

• Added .gitignore (ignores result symlink)
• Removed stale result symlink from repo
• Updated nix/configuration.nix to export flakeModules for consumers
• Updated all test flake.lock files