Deprecate the migrate command

[Copilot]

The migrate command needs to signal to users that it is on the path to removal, without breaking existing workflows.

Changes

• Deprecation warning: migrate() now emits a logger.warning at invocation time before performing any work:

  ⚠️  The 'migrate' command is deprecated and will be removed in a future release.
  

• Test coverage: Added a log_sink fixture and test_migrate_shows_deprecation_warning to assert the warning is present in log output when migrate is called.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Deprecate the migrate command</issue_title>
<issue_description>Print a warning message but still don't change the functionality</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Deprecate the migrate command #376

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.