chore: exclude the SDD scratch workspace from prettier

.superpowers/ holds git-ignored controller artifacts (briefs, reports,
review packages). Prettier still walked it, so format:check — and with it
check:production — failed on scratch markdown.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.prettierignore

@@ -26,3 +26,6 @@ focus-shims.d.ts
 **/focus-shims.d.ts
 tsconfig.focus.json
 **/tsconfig.focus.json
+
+# SDD scratch workspace (git-ignored controller artifacts)
+.superpowers/