Use golangci-lint fmt to format code (#163)

Use `golangci-lint fmt` to format code, upgrading `.golangci.yml` to v2 and mirroring the linter configuration used by https://github.com/go-gitea/gitea. `gci` now handles import ordering into standard, project-local, blank, and default groups.

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

Changes:
- Upgrade `.golangci.yml` to v2 format with the same linter set as gitea (minus `prealloc`, `unparam`, `testifylint`, `nilnil` which produced too many pre-existing issues)
- Add path-based exclusions (`bodyclose`, `gosec` in tests; `gosec:G115`/`G117` globally)
- Run lint via `make lint-go` in CI instead of `golangci/golangci-lint-action`, matching the pattern used by other Gitea repos
- Apply safe auto-fixes (`modernize`, `perfsprint`, `usetesting`, etc.)
- Add explanations to existing `//nolint` directives
- Remove dead code (unused `newRemoteReusableWorkflow` and `networkName`), duplicate imports, and shadowed `max` builtins
- Replace deprecated `docker/distribution/reference` with `distribution/reference`
- Fix `Deprecated:` comment casing and simplify nil/len checks

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

Reviewed-on: https://gitea.com/gitea/act/pulls/163
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

Makefile

@@ -13,6 +13,8 @@ endif
 
 ACT ?= go run main.go
 
+GOLANGCI_LINT_PACKAGE ?= github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.11.4
+
 HAS_TOKEN = $(if $(test -e ~/.config/github/token),true,false)
 ifeq (true,$(HAS_TOKEN))
 	export GITHUB_TOKEN := $(shell cat ~/.config/github/token)
@@ -27,11 +29,19 @@ build:
 
 .PHONY: format
 format:
-	go fmt ./...
+	go run $(GOLANGCI_LINT_PACKAGE) fmt
+
+.PHONY: format-check
+format-check: format
+	@diff=$$(git diff --color=always); \
+	if [ -n "$$diff" ]; then \
+		echo "Please run 'make format' and commit the result:"; \
+		printf "%s" "$${diff}"; \
+		exit 1; \
+	fi
 
 .PHONY: format-all
-format-all:
-	go fmt ./...
+format-all: format
 	npx prettier --write .
 
 .PHONY: test
@@ -41,7 +51,7 @@ test:
 
 .PHONY: lint-go
 lint-go:
-	golangci-lint run $(FIX)
+	go run $(GOLANGCI_LINT_PACKAGE) run $(FIX)
 
 .PHONY: lint-js
 lint-js: