feat(runner): task dependencies (depends)

[CalvinAllen]

Description

Run prerequisite tasks before a task executes.

Proposed Syntax

deploy:
  depends: [build, test]
  cmd: ./deploy.sh

Acceptance Criteria

• Parse depends property as array of task names
• Execute dependencies before main task
• Skip already-run dependencies in the same session
• Detect circular dependencies

Open Questions

• Should dependencies run in parallel or sequential?
• How to handle dependency failures?