redesign: complete refactor

[mping]

Rewrite intemporal with ClojureScript support and improved architecture

This is a complete redesign of intemporal (v2), preserving the old
implementation in src2/ for reference.

Key changes:

API redesign:

• New intemporal.core namespace with simplified API
• stub, stub-protocol, start-workflow, resume-workflow
• make-workflow-engine and with-workflow-engine for lifecycle management
• Removed defn-workflow macro in favor of regular functions
• Use suspension mechanism instead of deferreds/promises

Architecture improvements:

• Modular internal/ structure (context, execution, runtime, activity, error)
• Clean protocol-based abstractions (IStore, IActivityExecutor, IScheduler)
• Observer pattern for monitoring and OpenTelemetry tracing
• Platform-specific execution engines for JVM and JavaScript

The old implementation is preserved in src2/ for backward compatibility
during migration.