feat: add strategy abstraction with pdca and pipeline strategies

skills/workflow-design/SKILL.md

@@ -25,6 +25,10 @@ ArcheFlow's PDCA cycles spiral upward through iterations — each cycle incorpor
 │         Plan (design)     ← Cycle 1 (initial)
 ```
 
+## Strategy vs Workflow
+
+A **strategy** defines the execution shape: PDCA is cyclic (Plan-Do-Check-Act with feedback loops), pipeline is linear (Plan-Implement-Review-Verify, no cycle-back). A **workflow** defines the depth: fast uses fewer agents and cycles, thorough uses more. Strategy and workflow are orthogonal — you can run a `fast` workflow with either strategy, though `thorough` always uses PDCA because linear flows cannot iterate on findings.
+
 ## Built-in Workflows
 
 ### `fast` — Single Turn