14bdc6fe68
Implements bd-caa9: migration tool for existing users to enable separate branch workflow. Features: - bd migrate --to-separate-branch <branch> sets sync.branch config - Validates branch names (no whitespace/empty) - Dry-run support with --dry-run flag - Idempotent (safe to run multiple times) - JSON output support - Clear next steps for users Closes bd-caa9, bd-a101 Amp-Thread-ID: https://ampcode.com/threads/T-93c3427d-12a1-4d9a-8690-1d0cfe22188f Co-authored-by: Amp <amp@ampcode.com>