Update nixica to 0.9.0

Summary

• Update nixica 0.3.0 → 0.9.0
• Adopt named-arg mkFlake API
• Move CI config to ci/ partition (ci/nix/ci.nix)
• Add ci/flake.nix with empty inputs (avoids self-referencing gitlab-ci cycle)
• Remove flake.lib option declaration (now provided by nixica's modular module)

Ref: #11

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