chore(integrations): remove deprecated gemini package

Deletes internal/integrations/gemini now that all callers have been
migrated to internal/integrations/ai. Keeps the codebase free of
dead code and eliminates the misleading provider-specific package name.

Closes #68

Signed-off-by: Kavirubc <hapuarachchikaviru@gmail.com>

Author: Kavirubc