feat(runner): parallel execution

[CalvinAllen]

Description

Implement parallel execution for parallel blocks within steps.

Acceptance Criteria

• Execute items in parallel block concurrently
• Wait for all parallel items to complete before continuing
• Handle output from parallel tasks (interleave or buffer)
• Report failures from parallel execution
• Decide on fail-fast vs. wait-for-all behavior

Example

build-all:
  steps:
    - cmd: echo "Starting builds"
    - parallel:
        - dir: services/api
          task: build
        - dir: services/web
          task: build
    - cmd: echo "Done"