feat: add and support json schemas

[franklinkim]

Description

Add JSON Schemas for NATS and Temporal migration files, and support an object body form ({"$schema": ..., "ops": [...]}) so editors can validate and autocomplete migrations inline.

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 💥 Breaking change
• 📝 Documentation
• ♻️ Refactoring
• 🏃‍➡️ Performance
• ✅ Tests
• 🔐 Security
• 🔧 Build/CI

Changes

• Add migration.schema.json for both database/nats and database/temporal, describing every op kind.
• run.go (both drivers) accepts an object body {"$schema", "ops": [...]} in addition to the existing top-level array.
• Add make schema target validating example migrations against the schemas; wire it into make check.
• Pin jsonschema tool in .mise.toml.
• Convert example migrations to the object form with $schema references.
• Document the object form in database/nats/doc.go and database/temporal/README.md.

Checklist

• My code adheres to the coding and style guidelines of the project.
• I have performed a self-review of my own code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.