Feat: export migrate method

[AxiosLeo]

Note

Medium Risk
Touches migration execution and error handling (replacing process.exit with thrown errors), which can change failure behavior for automation/CI and consumers relying on prior exit semantics.

Overview
Exports a new programmatic migrate(action, dir, options) API from index.js (typed in index.d.ts) so consumers can run DB migrations without invoking the CLI.

Refactors commands/migrate.js to call the exported migrate function, normalizes action to lowercase, adjusts port parsing defaults, and simplifies error reporting.

Updates migration failure handling in src/migration.js to throw errors instead of calling process.exit, and adds workspace/build config (build script, pnpm-workspace.yaml) plus devDependency cleanup.

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