Use golangci-lint fmt to format code (#178)

Use `golangci-lint fmt` to format code, replacing the previous gofumpt-based formatter. https://github.com/daixiang0/gci is used to order the imports.

Mirrors https://github.com/go-gitea/gitea/pull/37194.

---
This PR was written with the help of Claude Opus 4.7

Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/178
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

Makefile

@@ -3,9 +3,8 @@ EXECUTABLE := gitea-mcp
 VERSION ?= $(shell git describe --tags --always | sed 's/-/+/' | sed 's/^v//')
 LDFLAGS := -X "main.Version=$(VERSION)"
 
-GOLANGCI_LINT_PACKAGE ?= github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.10.1
+GOLANGCI_LINT_PACKAGE ?= github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.11.4
 GOVULNCHECK_PACKAGE ?= golang.org/x/vuln/cmd/govulncheck@v1
-GOFUMPT_PACKAGE ?= mvdan.cc/gofumpt@v0.9.2
 
 .PHONY: help
 help: ## print this help message
@@ -51,7 +50,16 @@ dev: air ## run the application with hot reload
 
 .PHONY: fmt
 fmt: ## format the Go code
-	$(GO) run $(GOFUMPT_PACKAGE) -w .
+	$(GO) run $(GOLANGCI_LINT_PACKAGE) fmt
+
+.PHONY: fmt-check
+fmt-check: fmt ## check that Go code is formatted
+	@diff=$$(git diff --color=always); \
+	if [ -n "$$diff" ]; then \
+		echo "Please run 'make fmt' and commit the result:"; \
+		printf "%s" "$${diff}"; \
+		exit 1; \
+	fi
 
 .PHONY: lint
 lint: lint-go ## lint everything