chore(ci): remove sync-back workflows and align git workflow docs

[fleeser]

Summary

This PR removes workflow automation that rewrote branch history and updates our documented Git process to a PR-first merge-commit strategy for long-lived branches.

Changes

• Deleted:
  • .github/workflows/auto-merge-release-please.yml
  • .github/workflows/backsync-production-to-staging.yml
  • .github/workflows/backsync-staging-to-development.yml
• Updated README.md:
  • Clarified branch strategy for development, staging, and production
  • Documented PR-first flow for all promotions
  • Documented merge method rules (Create a merge commit) and what to avoid (Rebase and merge for promotions)

Why

The removed sync-back/rebase automation caused commit hash drift between long-lived branches and made promotions harder to reason about.
The updated process keeps branch operations explicit and consistent with manual PR-based promotions.

Validation

• Verified branch now contains only the intended workflow deletions + README update.
• No runtime code path changes (CI/workflow config + documentation only).