chore: Update default branch references to main

[jbeckwith-oai]

Summary

Updates repo-owned references from master to main after the default branch rename.

Changes

• Updates the Create releases workflow trigger and branch guard to use main.
• Updates OpenAI repo documentation links from blob/master / tree/master to blob/main / tree/main.
• Leaves the vendored zod source comment unchanged because it points to an external repository.

Validation

• git diff --check
• Parsed .github/workflows/create-releases.yml with Ruby YAML loader.