[Workflow] Implement parallel task execution with thread-safe repositories

[thinkclay]

Type: Enhancement

Priority: High

Location: crates/radium-core/src/workflow/parallel.rs:64

Description:
Currently, parallel workflow execution uses a sequential execution with parallel facade. True parallel execution requires refactoring repositories to Arc<dyn Repository + Send + Sync>.

Current Behavior:

• Workflow tasks execute sequentially
• Parallel executor exists but delegates to sequential implementation
• Repository trait doesn't implement Send + Sync bounds

Expected Behavior:

• Tasks should execute in parallel when dependencies allow
• Repositories should be thread-safe and shareable across tasks
• Proper error handling for concurrent execution

Dependencies:

• Requires Repository trait refactoring to add Send + Sync bounds
• Need Arc-wrapped repositories for shared access
• May require async-aware synchronization primitives

Acceptance Criteria:

• Repository trait implements Send + Sync
• Repositories wrapped in Arc for shared access
• Parallel executor properly spawns tasks concurrently
• Tests verify parallel execution behavior
• Error handling works correctly in concurrent context

Related: Part of workflow execution improvements from TODO_TRACKING.md

Priority: HIGH