Update migrations skill for direct calling syntax

[mikecann]

Summary

• Update the Convex migration helper to recommend calling migrations.define exports directly with npx convex run migrations:myMigration.
• Keep migrations.runner() documented for general-purpose runners and reusable ordered series.
• Add direct-call examples for production runs, dry runs, reset, and ad hoc next series.

Validation

• npm run format-check
• npm run test:skills-manifest

Notes

Verified the syntax against @convex-dev/migrations@0.3.4, whose README now describes direct CLI invocation as the simplest way to run a migration.