feat(workflows): Remove DurableTask dependency

## Summary

Rewrite the Dapr JS SDK Workflow implementation to communicate directly with the Dapr sidecar's Workflow gRPC API, removing the current dependency on the DurableTask framework internals.

## Background

The current workflow implementation in the JS SDK is built on top of an internal `durabletask` package (`src/workflow/internal/durabletask/`) that directly implements the DurableTask protocol — including its own orchestration replay logic, protobuf definitions, task hub worker, and gRPC transport. This approach:

- Couples the SDK tightly to DurableTask implementation details (proto definitions, replay semantics, orchestration state machine)
- Requires maintaining a complex internal framework that duplicates logic already handled by the Dapr sidecar
- Makes it harder to adopt new Dapr Workflow API features as they are added at the sidecar level
- Increases the SDK's surface area and maintenance burden significantly

## Proposed Change

Rewrite the workflow subsystem to use the Dapr sidecar's native Workflow gRPC API directly (the same API used by `DaprWorkflowClient` for management operations). This means:

1. **Remove `src/workflow/internal/durabletask/`** — the orchestration replay engine, proto files, task hub worker, and all DurableTask-specific types
2. **Implement workflow runtime against the Dapr Workflow API** — register workflows and activities via the sidecar's streaming API, receive work items, and report completions through the sidecar
3. **Preserve the public API** — `WorkflowRuntime`, `WorkflowContext`, `WorkflowActivityContext`, `DaprWorkflowClient`, and all user-facing types should remain backward-compatible
4. **Use ConnectRPC/gRPC** consistent with the rest of the SDK's client infrastructure

## Current Architecture
WorkflowRuntime → TaskHubGrpcWorker → DurableTask gRPC protocol → Dapr sidecar WorkflowContext → OrchestrationContext (replay engine)

## Target Architecture
WorkflowRuntime → Dapr Workflow gRPC API → Dapr sidecar WorkflowContext → Lightweight wrapper over sidecar-managed state

## Acceptance Criteria

- [ ] `src/workflow/internal/durabletask/` is removed
- [ ] Workflow runtime communicates with Dapr sidecar via the native Workflow API
- [ ] All existing public API types and method signatures remain backward-compatible
- [ ] Existing workflow e2e tests pass without modification (or with minimal test infrastructure changes)
- [ ] Proto file copying in build scripts (`build-copy-workflow-proto`) is no longer needed
- [ ] Package size is reduced

## Dependencies

- Dapr sidecar Workflow API must support all operations currently used by the DurableTask worker (work item streaming, completion reporting, timer scheduling, external event handling, sub-orchestration, continue-as-new)