Add role-local migrated skill guidance

packages/codebase-design/SKILL.md

@@ -0,0 +1,15 @@
+---
+name: codebase-design
+description: Examine module interfaces and seams while drafting a repository Plan.
+metadata:
+  relayview-display-name: Codebase design
+  relayview-version: 0.3.0
+---
+
+# Codebase design for Planner
+
+Use this guidance while deciding the implementation boundary of an assigned Plan. Read the repository's current architecture, relevant code and tests before proposing a module change. Use the [module guide](module-guide.md) to compare interfaces and identify the smallest seam that can carry the requested behavior.
+
+Describe the existing interface, the behavior hidden behind it, and the caller obligations that a change would add or remove. Compare at least two plausible designs in the Plan when the seam is consequential. Record a recommendation with the trade-off and a testable observation, rather than treating a preferred abstraction as verified behavior.
+
+Submit the result through the assigned planning interface. Keep code edits, branch operations, subagent scheduling, and workflow transitions with their existing owner-controlled roles and stages.