feat: add flake-aware Nix language support

[tirth8205]

Summary

• Adds Nix language parsing support (flake-aware: flake.nix inputs produce IMPORTS_FROM edges to GitHub flake URLs)
• Parser handles import/callPackage expressions and top-level attribute-set bindings
• Adds TestNixParsing test class covering language detection, binding extraction, flake-input edges, and CONTAINS wiring
• Adds Nix fixtures (tests/fixtures/sample.nix, tests/fixtures/sample_module.nix)

This PR replaces #412 (BrockLeeBytes/code-review-graph → feat: add flake-aware Nix language support), which had merge conflicts with main. All original Nix changes are preserved; conflict in tests/test_multilang.py was resolved by keeping both the new TestNixParsing class and the Spring/Temporal/Kafka test classes added by main.

Test plan

• uv run pytest tests/test_multilang.py -k TestNix -v passes
• uv run pytest tests/test_parser.py -v passes
• uv run ruff check code_review_graph/ clean
• Full CI matrix (3.10–3.13) passes

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