fix: add golangci-lint to pre-commit hook, clarify R2_ENDPOINT in env example

Pre-commit now runs golangci-lint (3m timeout) after go vet, before staticcheck. R2_ENDPOINT comment updated to clarify it is prod-only and local dev uses S3_* vars.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

.githooks/pre-commit

@@ -41,6 +41,13 @@ if [ -n "$BACKEND_FILES" ]; then
 	fi
 	cd ..
 
+	printf "${YELLOW}Checking golangci-lint...${NC}\n"
+	if ! cd backend && golangci-lint run ./... --timeout 3m 2>&1; then
+		printf "${RED}✖ golangci-lint failed${NC}\n"
+		FAILED=1
+	fi
+	cd ..
+
 	printf "${YELLOW}Checking staticcheck...${NC}\n"
 	if ! cd backend && staticcheck -tags "test,dev" ./... 2>&1; then
 		printf "${RED}✖ staticcheck failed${NC}\n"