Add an `earth migrate` command to assist migration from earthly

[kmannislands]

Summary

Suggested during review of the migration guide (PR #14): add a migrate subcommand to the earth CLI to help users move from earthly to EarthBuild automatically.

Motivation

The migration guide documents a number of mechanical changes users must make by hand:

• Rename the binary reference earthly → earth in scripts/CI
• Update EARTHLY_* → EARTH_* environment variables (deprecated in v0.8.x, removed in v0.9.x)
• Rename ARG EARTHLY_* → ARG EARTH_* built-in args in Earthfiles
• Remove the PROJECT command from Earthfiles
• Drop removed flags (--auto-skip, --satellite, --auth-token, …)
• Update docker.io/earthly/* image references to docker.io/earthbuild/*

A earth migrate command could detect and optionally auto-apply these transformations (e.g. rewriting Earthfiles and flagging config/CI changes), easing adoption ahead of the v0.9.x breaking release.

Notes

• Raised by @janishorsts. Acknowledged concern: this is a larger effort and contributor bandwidth is limited, so this is a nice-to-have / help-wanted rather than release-blocking.
• Related outstanding migration work: Print warning when EARTHLY_ prefixed env is used #507 (env-var deprecation warnings), Support EARTH_ built-in ARGs and warn on EARTHLY_ built-in ARG usage #630 (built-in ARG support + warnings).