AGENTS.md: sync Unix-only provider list with windows_compat frozenset

After adding chromewebstore and gem to
UNIX_ONLY_PROVIDER_NAMES in windows_compat.py, the parenthetical
in AGENTS.md fell behind — it still listed only the original 7
providers. Bring the prose in sync with the actual frozenset so
readers know those two providers are also skipped on Windows.

Flagged by devin-ai-integration review.

Author: claude

AGENTS.md

@@ -92,7 +92,8 @@ effects are correct.
 - assume ALL binproviders (other than apt on macos) are always available in the host environment (e.g. brew, pip, npm, docker, gem, etc. are ALL available in all environments), let it hard fail naturally if any are missing/broken. do not skip or disable those failing tests.
 - Exception for Windows: the Unix-only providers listed in
   `abxpkg.windows_compat.UNIX_ONLY_PROVIDER_NAMES` (apt / brew / nix /
-  bash / ansible / pyinfra / docker) have no Windows implementation, so
+  bash / ansible / pyinfra / docker / chromewebstore / gem) have no
+  Windows implementation, so
   `tests/conftest.py::pytest_collection_modifyitems` skips their per-file test
   modules on Windows. Every other provider must still run its real
   install lifecycle on Windows and fail loudly if the host tooling is