PR 6: Add expect field to engine for command step assertions

[5uck1ess]

Summary

Engine enhancement from the deterministic workflow conversion project.

Adds an expect field to command steps that controls exit code semantics:

- id: repro
  command: "npm test"
  expect: failure  # step fails if exit code is 0

Value	Behavior
success (default)	Non-zero exit is informational, output captured
failure	Step fails if exit code is 0 (command unexpectedly succeeded)

Use case: Bugfix reproduction gates. The repro step must fail (bug exists) before the fix step runs, then the verify step must succeed (bug fixed).

Changes

• src/engine/workflow.go — Add Expect field to WfStep, validation (command-only, success/failure values)
• src/engine/engine.go — Check expect condition after command execution
• src/engine/engine_test.go — 6 new tests covering parse, validate, and runtime behavior
• skills/creating-workflows/SKILL.md — Document the expect field

Test results

ok  github.com/5uck1ess/devkit/engine  1.720s
ok  github.com/5uck1ess/devkit/loops   4.047s

All existing tests pass. 6 new tests added.

Test plan

• expect: failure with non-zero exit → step succeeds (repro confirmed)
• expect: failure with zero exit → step fails with "expected failure" error
• No expect field → default behavior unchanged (non-zero is informational)
• expect on prompt step → validation error
• expect: maybe → validation error (invalid value)
• Full suite: go test ./... passes