Add database migrations and CLI help

[rtcoder]

Summary

• add database migration base class, runner, repository tracking, and migrate/status/rollback commands
• add create:migration generator with a file-based stub
• make help list registered commands and show command-specific usage
• centralize CLI command discovery in a shared command registry used by the dispatcher and help command
• normalize CLI command names across colon, dash, underscore, and class-like forms
• document migrations, CLI help, and command discovery in README, developer docs, and refactoring notes

Tests

• composer validate --no-check-publish
• php -l shift
• find src application tests -name '*.php' -print0 | xargs -0 -n1 php -l
• composer test
• ./shift help
• ./shift help migrate:status
• ./shift help migrate-status
• ./shift help migrate_status
• DB_CONNECTION=sqlite DB_DATABASE=:memory: ./shift migrate:status
• DB_CONNECTION=sqlite DB_DATABASE=:memory: ./shift migrate
• DB_CONNECTION=sqlite DB_DATABASE=:memory: ./shift migrate:rollback
• DB_CONNECTION=sqlite DB_DATABASE=:memory: ./shift migrate-status
• ./shift health