fix(gemini-adapter): detect new ~/.gemini/oauth_creds.json auth path

[abigail-atheryon]

Summary

gemini-cli >=0.30 stores OAuth credentials at ~/.gemini/oauth_creds.json instead of the legacy ~/.config/gemini/ directory. The benchmark adapter's availability check still only looks at the legacy path, so users who have authenticated with a recent gemini-cli see NOT READY — No Gemini auth found even though the CLI itself works fine.

This patch accepts either path, so older and newer installs both succeed.

Repro before fix

$ gemini   # cached credentials load fine
$ ~/.claude/skills/gstack/bin/gstack-model-benchmark --prompt unused --models gemini --dry-run
  gemini: NOT READY — No Gemini auth found.

~/.config/gemini/ does not exist; ~/.gemini/oauth_creds.json does.

After fix

  gemini: OK

Verified end-to-end with a real benchmark run (gpt + gemini, 11.5s, no auth errors).

Test plan

• Dry-run shows gemini: OK on a host with only ~/.gemini/oauth_creds.json
• Real benchmark run completes without auth errors
• Verify legacy ~/.config/gemini/ path still detected (unchanged code path; trusting OR semantics)

🤖 Generated with Claude Code

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[garrytan]

Thanks @abigail-atheryon — your fix shipped in a prior wave with credit in the CHANGELOG. Closing since it's already on main. Appreciate the contribution.