Fix golangci-lint errcheck warnings; add make fixstyle

errcheck flagged six unchecked Close() calls. The production write
path in indexgen.ProcessDir now closes the index file explicitly
after Execute and surfaces the close error so a deferred flush
failure isn't lost; tests use _ = f.Close() since they don't care.

Also adds a 'make fixstyle' target that runs gofmt -w over the tree,
matching the convention used elsewhere.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Makefile

@@ -15,7 +15,7 @@ LDFLAGS      := -s -w \
 # beyond switching net/dns to the pure-Go resolver.
 export CGO_ENABLED := 0
 
-.PHONY: help all build build-amd64 build-arm64 test pkg-deb fmt vet lint check clean
+.PHONY: help all build build-amd64 build-arm64 test pkg-deb fmt fixstyle vet lint check clean
 
 help: ## Show this help
 	@printf "Usage: make <target>\n\nTargets:\n"
@@ -45,6 +45,9 @@ pkg-deb: build-amd64 build-arm64 ## Build .deb packages for amd64 and arm64
 fmt: ## go fmt everything
 	go fmt ./...
 
+fixstyle: ## Reformat the entire tree with gofmt
+	gofmt -w .
+
 vet: ## go vet everything
 	go vet ./...