chore(nix): use UnstoppableMango/nix-systems flake module

[UnstoppableMango]

The systems input now points at UnstoppableMango/nix-systems, and the flake imports its flake module so Hercules CI builds only x86_64-linux.
The previous input was nix-systems/default, so x86_64-darwin is no longer offered.
The flake module is imported from flake.nix alongside ./nix, which keeps systems = import inputs.systems; in nix/default.nix.
nix flake check --no-build passes, and nix eval .#herculesCI.ciSystems --json prints ["x86_64-linux"].

🤖 Generated with Claude Code

Summary by CodeRabbit

• Chores
  • Updated system compatibility configuration to use the project’s maintained systems definition.
  • Revised flake configuration loading to provide more consistent system-specific outputs.

[coderabbitai]

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⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Advanced

Run ID: 1229523e-9541-47b6-936b-c573957795a8

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Advanced

Run ID: 6c64956b-8b5f-4988-8755-afeb7cc73dce

📥 Commits

Reviewing files that changed from the base of the PR and between c7b0f53 and 234285a.

⛔ Files ignored due to path filters (1)

• flake.lock is excluded by !**/*.lock

📒 Files selected for processing (1)

• flake.nix

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📝 Walkthrough

Walkthrough

The flake now uses UnstoppableMango/nix-systems and configures flake-parts with the systems flake module and the local ./nix module.

Changes

Flake configuration

Layer / File(s)	Summary
Systems input and flake-parts wiring flake.nix	The systems input now points to github:UnstoppableMango/nix-systems. The outputs configuration imports inputs.systems.flakeModule and ./nix through flake-parts.lib.mkFlake.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Other

Merge Risk: ⚪ Minimal · up to 23428

The integration preserves the available systems while restricting Hercules CI to x86_64-linux, with no identified merge-blocking risk.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: updating Nix to use the UnstoppableMango/nix-systems flake module.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch nix/nix-systems-module

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