Improve parallel execution discoverability and add shorthand syntax

[tombee]

Summary

• Add parallel: shorthand syntax for cleaner workflow definitions
• Add security validations (nesting depth 3 levels, array size 10k, max_concurrency 1-100)
• Rewrite docs/features/parallel.md documentation with comprehensive examples
• Add integration tests for parallel execution with timestamp overlap verification

Changes

• pkg/workflow/definition.go: Add parallel shorthand parsing in UnmarshalYAML()
• pkg/workflow/validate.go: Add ValidateParallelNestingDepth(), ValidateForeachArraySize(), max_concurrency validation
• pkg/workflow/executor.go: Add runtime array size check in executeForeach()
• docs/features/parallel.md: Complete rewrite with shorthand syntax, all options documented
• docs/features/loops.md: Add cross-reference to parallel.md

Test plan

• All workflow package tests pass
• Security validation tests verify limits (nesting depth, array size, max_concurrency)
• Integration tests verify concurrent execution with timestamp overlap
• Shorthand syntax produces identical behavior to explicit syntax

Closes SPEC-8