Add build script, AGENTS.md, fix formatting

- scripts/build: gofmt, go vet, go build, docker build
- AGENTS.md: agents must run ./scripts/build before committing
- Fix gofmt issues in handler/auth.go and store/store.go

handler/auth.go

@@ -136,6 +136,7 @@ func (h *AuthHandler) ServeSignout(w http.ResponseWriter, r *http.Request) {
 	})
 	http.Redirect(w, r, "/signin", http.StatusSeeOther)
 }
+
 type contextKey string
 
 const userIDKey contextKey = "userID"