Add migration guide for consumer repositories

[coisa]

Objective

Add a migration guide for consumer repositories adopting Fast Forward DevTools.

Current Limitation

The project documents installation and sync commands, but consumer maintainers would benefit from a guided migration path that explains what will change, how to review those changes, and how to roll out automation safely.

Proposed Work

Create a step-by-step migration guide for taking an existing PHP package repository from no dev-tools integration to synchronized Fast Forward automation.

Scope

• Preflight checks before installing dev-tools.
• Composer installation command and expected package changes.
• Running dev-tools:sync safely.
• Reviewing generated workflows, .editorconfig, Dependabot, wiki submodule, skills, license, Git hooks, .gitignore, and .gitattributes changes.
• Handling branch protection and bot commits.
• Suggested PR rollout strategy for consumer repositories.

Non-goals

• Replacing the quickstart.
• Covering non-PHP projects.
• Automating migration in this issue.

Acceptance Criteria

Delivery Criteria

• A consumer migration guide exists under docs/usage or docs/getting-started.
• The guide explains expected file changes from sync.
• The guide includes a safe review workflow for maintainers.
• The guide references relevant command docs.
• The guide covers how to handle existing custom workflows and config files.

Architectural / Isolation Criteria

• The guide builds on existing docs rather than duplicating full command references.
• Examples use Fast Forward repository conventions consistently.