feat(pipeline): add optional step support for non-blocking failures

[nextlevelshit]

Summary

• Add optional: true field on pipeline step definitions allowing steps to fail without halting the pipeline
• Introduce StateFailedOptional state across pipeline executor, event emitter, display layer, and state store
• Gracefully skip dependent steps when their optional dependency fails, while independent steps continue
• Support optional step handling in both Execute and ResumeFromStep code paths
• Display optional failures with distinct warning styling (⚠ icon, yellow color) in the TUI

Spec

See specs/118-optional-pipeline-steps/spec.md for the full feature specification including user stories, functional requirements, success criteria, and edge cases.

Test plan

• Unit tests: 7 new tests covering optional step behavior:
  • TestOptionalStepFailureContinuesPipeline — optional step fails, pipeline continues
  • TestOptionalStepDependencySkipping — dependent steps skipped when optional dep fails
  • TestNonOptionalStepFailureHaltsPipeline — backward compatibility: non-optional still halts
  • TestOptionalStepDefaultsFalse — Optional defaults to false
  • TestPipelineCompletionWithOptionalFailures — completion message includes optional failure count
  • TestStateFailedOptionalConstant — state constant verification
  • TestShouldSkipDueToOptionalDep — table-driven tests for dependency skip logic
• YAML parsing tests: Verify optional: true/false/absent parses correctly
• Race detector: Full suite passes with go test -race ./...
• All existing tests continue to pass (backward compatible)

Known limitations

• Resume does not reconstruct failedOptionalSteps map from prior run state (steps re-execute on resume)
• No manifest-level validation that optional is only used on leaf or non-critical steps (left to user)

Closes #118