Require Docs Agent PRs for bootstrap flows

[chubes4]

Summary

• require the Docs Agent bootstrap flows to open a documentation PR in the consumer proof workflow
• keep maintenance flows able to pass with no_changes
• make the prompt completion instruction match the selected lifecycle mode

Testing

• Fresh main run without this guard passed incorrectly as no_changes after writing a docs commit without a PR: https://github.com/Automattic/agents-api/actions/runs/25637756735

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Diagnosed the missing bootstrap guard on the merged workflow path and prepared this CI workflow/script correction; Chris remains responsible for review and merge.