Add prototype skill and related documentation for interactive design exploration

skills/engineering/to-issues/SKILL.md

@@ -64,6 +64,8 @@ A reference to the parent issue on the issue tracker (if the source was an exist
 
 A concise description of this vertical slice. Describe the end-to-end behavior, not layer-by-layer implementation.
 
+Avoid specific file paths or code snippets — they go stale fast. Exception: if a prototype produced a snippet that encodes a decision more precisely than prose can (state machine, reducer, schema, type shape), inline it here and note briefly that it came from a prototype. Trim to the decision-rich parts — not a working demo, just the important bits.
+
 ## Acceptance criteria
 
 - [ ] Criterion 1