Migrate to nixica flake with standard CI template

Summary

• Convert quality-control/policy to a proper nixica flake
• Replace bespoke .gitlab-ci.yml with the standard remote template
• Add dev/ partition using nixica.lib.mkPartitionModule
• Add nix/modules/flake/policy-pdf.nix to build the Typst PDF as a nix package (nix build .#policy)
• Add "Repository Standards" and "Nixica" sections to policy document
• Document how mkFlake, mkPartitionModule, and import-tree work
• Enable nixica formatting (deadnix, nixfmt, statix, typos)

@locallycompact