Add conditional step execution in YAML workflows

[tombee]

Problem

YAML workflows currently execute all defined steps unconditionally. Real workflows need branching logic based on step results or inputs.

Concrete example from foreman: The implementing stage should skip test running if no test files exist, or branch to different strategies based on codebase type.

Proposal

Add conditional execution support:

steps:
  - id: check_tests
    shell.run: "test -d tests && echo 'has_tests' || echo 'no_tests'"
    
  - id: run_tests
    if: "{{.steps.check_tests.stdout}} == 'has_tests'"
    shell.run: "npm test"
    
  - id: skip_message
    if: "{{.steps.check_tests.stdout}} == 'no_tests'"
    type: llm
    prompt: "No tests to run, proceeding with implementation"

General Utility

1. Conditional workflows - Branch based on analysis results
2. Error handling - Skip subsequent steps if earlier step fails
3. Feature detection - Adapt workflow to environment capabilities

Acceptance Criteria

• if field on steps for conditional execution
• Template expressions evaluate to boolean
• Falsy conditions skip step (not error)
• Skipped step outputs are empty/null
• Support comparison operators (==, !=, contains)
• Integration test for conditional branching

Priority

Medium - Improves workflow flexibility, not blocking for MVP.

Labels

enhancement, sdk, priority:medium

[tombee]

Implemented in PR #12 (SPEC-7)