Checkpoints V2: Update migration completion message

[computermode]

https://entire.io/gh/entireio/cli/trails/248

Add more helpful information to the migration completion message so folks know where to see the logs and metadata if desired.

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Note

Low Risk
Low risk: only adjusts CLI post-migration messaging and extracts it into a helper, with no changes to migration logic or data writes.

Overview
Improves the checkpoints v2 migration completion output by extracting it into printMigrateCheckpointsV2Completion and adding guidance on where v2 checkpoints live and how to inspect them.

The completion message now explains that v2 checkpoints are stored as custom refs under refs/entire/checkpoints/v2/*, provides a git ls-remote command to list them, and points users to entire.io session logs for viewing log files and metadata.

^{Reviewed by Cursor Bugbot for commit ddfb873. Configure here.}

[Copilot]

Pull request overview

Updates the hidden migrate --checkpoints v2 command’s completion output to provide more actionable guidance about where v2 checkpoint data lives and how to inspect it.

Changes:

• Replaces the inline migration-complete fmt.Fprintf with a dedicated printMigrateCheckpointsV2Completion helper.
• Expands the completion message with notes about v2 checkpoint refs and suggested inspection commands / UI pointers.