feat(runner): conditional execution (if/when)

[CalvinAllen]

Description

Run tasks conditionally based on environment or file conditions.

Proposed Syntax

deploy:
  if: \${{ env.CI == 'true' }}
  cmd: ./deploy.sh

test:
  when:
    - file_exists: package.json
  cmd: npm test

Acceptance Criteria

• Support environment variable conditions
• Support file existence checks
• Support expression syntax
• Clear skip message when condition not met