feat(run_config): Gemini via Google's Antigravity CLI, all three tracks

[renxiongscale]

Adds gemini-3p6-flash_antigravity.sh to run_config/{qa,tw,rf}/, following
the existing opus-4p6_claude-code.sh pattern. This runs Gemini under Google's
own agent scaffold (agy) rather than Claude Code or mini-swe-agent — the
comparison most people benchmarking a Gemini model here actually want.

Each track keeps its own harness settings: keepalive for qa/tw,
modal_vm_runtime for rf, and the allowlisted agent host for tw
(generativelanguage.googleapis.com).

Two caveats, documented in the README rather than left to be rediscovered

• Harbor's antigravity-cli agent needs
  harbor#2780 first.
  Without it, settings are written to ~/.agy/antigravity-cli/ while agy
  reads ~/.gemini/antigravity-cli/; GEMINI_API_KEY is forwarded but ignored,
  so the run falls through to an interactive login that hangs headless; and the
  trajectory hook looks for session-*.jsonl files agy 1.1.13 no longer
  writes, so runs come back with no trajectory at all.
• agy rejects model ids outside its client-side allowlist before any request
  leaves the process. A --model failure there is not a quota or auth
  problem, and which ids are accepted depends on your key's entitlement. Hence
  gemini-3.6-flash in the committed configs — it works with an ordinary key.

Context

I ran all three leaderboards with this scaffold (284/284 tasks). I'm not
proposing those numbers for the leaderboard: our verifier ran inside the Modal
sandbox, where our internal LiteLLM host doesn't resolve, so it fell back to
Gemini grading its own answers. That is not comparable to the Claude Opus 4.5
judge task.toml defaults to. Mentioning it only to say the configs are
exercised end to end, not written from the docs.

Happy to add a mswea-style yaml or adjust -n/-k to match your house
defaults — I picked conservative values because agy treats an upstream 429 as
fatal and abandons the run rather than backing off.

Draft pending a maintainer's read on whether you want a Gemini config in-tree at
all.

🤖 Generated with Claude Code

Greptile Summary

Adds Antigravity CLI configurations for running Gemini 3.6 Flash across all three benchmark tracks and documents the required Harbor compatibility patches.

• Preserves each track’s existing execution settings and output layout.
• Supplies the Gemini API key and permits the Gemini API hostname.
• Documents current authentication, trajectory collection, work-directory, and model-allowlist limitations.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
README.md	Documents the new Antigravity configurations and the upstream Harbor limitations that affect their results.
run_config/qa/gemini-3p6-flash_antigravity.sh	Adds the QA Gemini run configuration with keepalive enabled.
run_config/rf/gemini-3p6-flash_antigravity.sh	Adds the Repository Fixing Gemini run configuration with Modal VM runtime enabled.
run_config/tw/gemini-3p6-flash_antigravity.sh	Adds the Test Writing Gemini run configuration with restricted-network agent-host configuration.

_{Reviews (2): Last reviewed commit: "docs: point at harbor#2776 and #2687 ins..." | Re-trigger Greptile}