Add `partio migrate` command for checkpoint v1-to-v2 format upgrades

[jcleira]

Summary

Add a partio migrate --checkpoints v2 CLI command that migrates existing checkpoint data from the v1 orphan branch format to the v2 compact format (compact transcript.jsonl + metadata on /main ref layout).

Why

Before checkpoints v2 can become the default, users with existing v1 checkpoint data need a migration path. Without this, adopting v2 means losing historical checkpoint data or maintaining two parallel formats indefinitely.

What to implement

• New partio migrate Cobra command with --checkpoints v2 flag
• --dry-run flag for previewing migration without writing
• Migration logic: read v1 branch commits → extract session/transcript data → write to v2 ref layout
• Idempotent execution (safe to run twice)
• Progress output showing migration status

Acceptance criteria

• partio migrate --checkpoints v2 converts existing v1 checkpoint data to v2 format
• Migration is idempotent
• --dry-run previews without writing
• All existing checkpoint metadata is preserved
• make test and make lint pass

Source

Inspired by entireio/cli#855 (Checkpoints V2 migration) and entireio/cli changelog 0.5.3.