Update docs for onboard and migrate-pr commands

- README: add onboard and migrate-pr to CLI reference
- Guide Step 5: add onboard as recommended Option A, move manual
  import/reset to Option B, document migrate-pr usage
- Guide "Adding a New Target": mention onboard as preferred path

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -68,6 +68,8 @@ josh-sync sync [--forward|--reverse] [--target NAME[,NAME]] [--branch BRANCH]
 josh-sync preflight
 josh-sync import <target>
 josh-sync reset <target>
+josh-sync onboard <target> [--restart]
+josh-sync migrate-pr <target> [PR#...] [--all]
 josh-sync status
 josh-sync state show <target> [branch]
 josh-sync state reset <target> [branch]