fix(packaging): let gateway auto-detect package driver

[drew]

Summary

Unset the packaged Debian and Homebrew OPENSHELL_DRIVERS=vm defaults so openshell-gateway uses its normal runtime auto-detection. The VM driver helper remains installed and discoverable for users who explicitly select OPENSHELL_DRIVERS=vm.

Related Issue

None.

Changes

• Remove OPENSHELL_DRIVERS=vm from the Debian user service.
• Remove OPENSHELL_DRIVERS: "vm" from generated Homebrew service environment variables.
• Update the Homebrew formula test and VM driver docs to reflect automatic driver selection.

Testing

• uv run pytest python/openshell/release_formula_test.py passes
• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)