Add _migrate_v1_to_v2() to convert aliases.json to config.json format.
Single-path-segment URLs are split into instance URL + default_database.
Auth keys are migrated from URLs to alias names. Original files are
backed up as .bak.
7 tests covering simple migration, no path segment, auth migration,
URL fallback for unmatched auth, skip if config exists, skip if no
aliases, and multi-path-segment URLs.
refs #29
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
