Feature Request: Support for Antigravity CLI (agy) as an external CLI provider

[jempolbagas]

Problem

Currently, the external CLI provider mechanism in LearnHarness is limited to Claude Code CLI (claude). Users who rely on the Google Antigravity ecosystem and have the Antigravity CLI (agy) authenticated locally cannot use their existing CLI sessions, subscriptions, or available models (such as gemini-4-argon and gemini-3-flash) within LearnHarness without managing separate direct API keys.

Proposal

Add support for Antigravity CLI (agy) alongside claude-code as an external CLI provider:

• Discovery & Verification: Detect agy from standard PATH and install directories, guarded by a minimum version check.
• Provider Settings: Add an opt-in toggle under Settings with installation/version status, disabled by default.
• Stream & Tool Execution: Invoke agy non-interactively using --input-format stream-json --output-format stream-json, bridging LearnHarness tools through an ephemeral MCP configuration (.agents/mcp_config.json) while restricting default agent tool execution.
• Model Catalog & Accounting: Expose supported Antigravity models in the catalog when the CLI is enabled and installed, categorized under CLI billing.

Questions

1. Does adding another external CLI agent align with the current roadmap for LearnHarness providers?
2. Are there specific design constraints or security boundaries around subprocess isolation and MCP bridging that an additional CLI integration should follow beyond what claude-code currently does?
3. Would the team prefer agy support to live directly in tree alongside claude-code, or is there plan to generalize the external CLI runner interface first?

[ZovutVanya]

Support that, if LearnHarness would be possible to use without the google's API, directly with a subscription, it would be great!