feat(core): refactor commands to use handler functions (Magek v2 Phase 1)

[javiertoledo]

Summary

This PR implements the first phase of Magek v2's data-oriented refactoring, focusing on commands. The key architectural change is moving from class-based command handlers to direct handler functions, enabling a DSL-first approach where decorators become thin wrappers.

Changes

• New DSL function: command() in packages/core/src/dsl/command.ts creates command metadata with handler functions
• Updated CommandMetadata: Uses handler function instead of class reference, adds name property
• Updated CommandDispatcher: Calls handler(input, register) directly instead of class.handle()
• Reimplemented @Command decorator: Now a thin wrapper around the DSL command() function
• Updated GraphQL infrastructure:
  • TargetTypeMetadata uses name instead of class
  • Added generateGraphQLTypeForMetadata() for metadata-based type generation
  • Added transform functions for CommandMetadata → TargetTypesMap

Breaking Changes

• CommandMetadata.class replaced with CommandMetadata.handler
• CommandMetadata now requires name property

Architecture (Before → After)

// Before (v1)
Decorators → Create metadata with class references → Runtime instantiates classes

// After (v2)
DSL Functions → Create pure data metadata → Runtime uses handlers directly
    ↑
Decorators (thin wrappers that call DSL functions)

Test plan

• All 327 tests in @magek/core pass
• Full monorepo build passes (rush build)
• Lint checks pass (rush lint:check)
• Full monorepo test suite passes (rush test)

Next Steps (Future PRs)

Following the incremental migration plan:

1. ✅ Commands (this PR)
2. Events - Add event() DSL, update EventMetadata
3. Entities + Reducers
4. Event Handlers
5. Read Models + Projections
6. Queries

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[chatgpt-codex-connector]

Preserve constructor side effects when wrapping @command classes

The decorator now builds the handler by Object.assign(Object.create(commandClass.prototype), input) and skips calling the class constructor, whereas the old path used createInstance() which does new instanceClass() and therefore runs constructor logic. Any existing command class that sets defaults, validates, or derives fields in its constructor will now receive an instance without those side effects, which can change command handling behavior at runtime. If constructors are intentionally supported today, consider instantiating with new commandClass() (or reusing createInstance) to keep parity with the previous behavior.

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