Warn when pages deploy runs without git checkout

[vliggio]

Summary

• Adds a warning when pages deploy or pages publish is run without a .git directory present in the working directory
• This prevents silent production deployments when actions/checkout is missing or failed

Fixes #373

Test plan

• Run pages deploy without actions/checkout and verify the warning appears
• Run pages deploy with actions/checkout and verify no warning
• Verify the deploy still proceeds (warning only, not a hard error)