This commit addresses intermittent cli-test failures on main caused by
GitHub API rate limiting when Nix attempts to fetch nixpkgs metadata.

Problem:
- Tests were hitting 403 errors with "API rate limit exceeded" message
- Despite having GITHUB_TOKEN configured in NIX_CONFIG and ~/.config/nix/nix.conf,
  the token wasn't being used properly by the Nix daemon on macOS runners

Root Cause:
- On macOS, the Nix daemon runs as a different user and reads /etc/nix/nix.conf
  instead of the user's ~/.config/nix/nix.conf
- The daemon wasn't picking up the access token configuration

Solution:
1. Configure /etc/nix/nix.conf system-wide on macOS runners and restart the daemon
2. As a backup, pass the access token explicitly via --option access-tokens in nix commands

Changes:
- .github/workflows/cli-tests.yaml: Update "Setup Nix GitHub authentication" step
  to configure /etc/nix/nix.conf on macOS and restart the daemon
- internal/nix/command.go: Add GITHUB_TOKEN to nix command args as --option access-tokens
- cli-tests-failure-report.md: Comprehensive diagnostic report of the issue

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

- Ensure /etc/nix directory and nix.conf file exist before appending
- Move configuration to AFTER Nix is installed in auto-nix-install job
- Configure user config before devbox runs (Nix installer respects this)
- Then configure system-wide config after Nix installation completes

This fixes the "No such file or directory" error in the auto-nix-install
job on macOS.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>