feat(allowlist): add Nix support

[ousamabenyounes]

Description

Adds Nix allowlist support by recognizing .nix files, registering a Nix-specific review rule, and covering case-insensitive extension matching in the allowlist test.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactoring (no functional changes)
• Documentation update
• CI / Build / Tooling

How Has This Been Tested?

• make test passes locally
• Manual testing (describe below)

Manual testing:

docker run --rm -v "$PWD":/src -w /src golang:1.26.5 go test ./internal/config/allowlist -run TestIsAllowedExt -count=1
docker run --rm -v "$PWD":/src -w /src golang:1.26.5 go test ./internal/config/... -count=1

Checklist

• My code follows the project's coding style (go fmt, go vet)
• I have performed a self-review of my code
• I have added tests that prove my fix is effective or my feature works
• New and existing unit tests pass locally with my changes
• I have updated the documentation accordingly (if applicable)
• I have signed the CLA

Related Issues

Fixes #614
Refs #470

[github-actions]

✅ OpenCodeReview: No comments generated. Looks good to me.

[lizhengfeng101]

Thanks for adding Nix support — the rule doc is thorough and well-structured!

One thing missing: whenever system_rules.json is updated, the documentation table in the review-rules pages needs to stay in sync (see PR #612 for the pattern). Please rebase onto the latest main and add a **/*.nix → nix.md row to each locale's table:

• pages/src/content/docs/en/review-rules.md
• pages/src/content/docs/zh-cn/review-rules.md
• pages/src/content/docs/ja/review-rules.md
• pages/src/content/docs/ko/review-rules.md
• pages/src/content/docs/ru/review-rules.md

The new row should go after the **/*.bicep entry and before the *(fallback) line, matching the declaration order in system_rules.json.