chore: drop premature SWINN_DESIGN briefs from the repo

These source briefs are exploratory and not part of the shipped
contract. Untrack them and ignore /docs/SWINN_DESIGN*.md so they stay
local-only.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: ikidnapmyself

.gitignore

@@ -16,3 +16,6 @@ Homestead.json
 Homestead.yaml
 npm-debug.log
 yarn-error.log
+
+# Premature design briefs — kept locally, not part of the repo
+/docs/SWINN_DESIGN*.md