feat(registry): add gemini-3-flash-agent

[arrrrny]

gemini-3-flash-agent is a great model and it has its own quota.
If there is no other reason not to list, better add it along with 3.1 flash-lite.

This is different quoata from the 3.1 quota

[github-actions]

This pull request targeted main.

The base branch has been automatically changed to dev.

[gemini-code-assist]

Code Review

This pull request adds three new model definitions—gemini-3-flash-agent, gemini-3-flash-lite, and gemini-3.1-flash-lite—to the registry, including their respective context lengths and thinking configurations. I have no feedback to provide.

[luispater]

Summary

Adds three Antigravity model definitions to the static registry (internal/registry/models/models.json):

• gemini-3-flash-agent
• gemini-3-flash-lite
• gemini-3.1-flash-lite

Key findings

• Data-only change (static model registry); no runtime code paths modified.
• Non-blocking: the PR title/body emphasize gemini-3-flash-agent, but the diff adds two Lite variants as well—consider updating the PR metadata for clarity.
• Non-blocking: please confirm the Lite variants’ thinking.levels and token limits match what Antigravity actually supports to avoid advertising unsupported reasoning levels.

Test plan

• Not run locally (CI is green).

This is an automated Codex review result and still requires manual verification by a human reviewer.