feat(init): offer to migrate project.md into config.yaml context

Problem: When you upgrade, openspec/project.md is kept but nothing reads it anymore. The only migration step is a one-time hint during openspec init, so if you miss it, your project conventions quietly stop reaching the agent.

Proposal: When init creates config.yaml and project.md exists, ask whether to copy project.md into context:. On yes, write the content there and still leave project.md in place for you to delete. In non-interactive runs, keep today's hint.

Out of scope: automatically deleting project.md, or summarizing its content.