feat(trail): restore create --checkout and --add-assignee, print the switch command

v0.12.0's project-trail create dropped the legacy --checkout and --add-assignee flags, so older scripts fail with an unknown flag, and a create on the base branch publishes a new branch but leaves the work on the base.

• --checkout switches to the published branch after the trail is printed; if git refuses, the error says the trail exists and gives the git switch command
• --add-assignee is accepted (hidden) and merged with --assignee
• without --checkout, create prints 'Next: git switch <branch>' when the linked branch is not the current one