feat: integrate Antigravity CLI (agy) harness support

[execsumo]

This PR adds full integration for the new Antigravity CLI (agy) agent harness in Skill Manager. All unit and integration tests are passing successfully.

[chzh12]

Thanks for the PR. I’m not comfortable approving this in the current form.

The main blocker is correctness: the updated test_skill_detail_orders_managed_locations_with_shared_store_first expectation inserts Antigravity before Codex in the location labels, but the following path assertion still expects that same index to be the Codex root. That makes the expected ordering/path mapping internally inconsistent, so I don’t want to approve this until the behavior and test are reconciled.

Please fix the location ordering expectation and confirm the intended managed-location order for Antigravity relative to the existing harnesses before this moves forward.