chore: limit Release Please commit batch

[jeremymcs]

Summary

• set Release Please commit-batch-size to 1 so it stops at the latest release SHA instead of prefetching older history
• add a regression assertion so the release config guard stays in place

Verification

• npm install
• node --import tsx --test server/appBranding.test.ts
• npm run check
• npm run test
• /simplify release-please-config.json server/appBranding.test.ts

[github-actions]

📄 Documentation Review Reminder

This PR changes source files but does not update any public documentation.

Changed source files:

server/appBranding.test.ts

Please check if documentation in docs/public/ needs to be updated:

• No documentation update needed
• Documentation updated in this PR
• Follow-up documentation issue created

Public docs are Markdown files in docs/public/. They are automatically
rendered to HTML and deployed to GitHub Pages when merged to main.

Available docs: getting-started.md, pr-babysitter.md, agent-dispatch.md, configuration.md, pr-questions.md