fix(integrations): migrate Antigravity (agy) layout to .agents/ and deprecate --skills (#2276)

* refactor(agy): update storage directory from .agent to .agents

* feat: update Antigravity integration to use .agents/ directory layout and add version compatibility warnings

* Apply suggestion from @Copilot

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* refactor: remove deprecated --skills flag from AgyIntegration and update related test assertions

* Update src/specify_cli/integrations/agy/__init__.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* refactor: update Antigravity integration requirement to v1.20.5 and remove obsolete tests

* test: update skills directory path from .agent to .agents in preset restoration test

* Update tests/integrations/test_integration_agy.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update tests/integrations/test_integration_agy.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>