chore: add golangci-lint, bump Go to 1.26, fix all lint issues (#133)

## Summary
- Add `.golangci.yml` with linter configuration matching the main gitea repo
- Add `lint`, `lint-fix`, `lint-go`, `lint-go-fix`, and `security-check` Makefile targets
- Add `tidy` Makefile target (extracts min Go version from `go.mod` for `-compat` flag)
- Bump minimum Go version to 1.26
- Update golangci-lint to v2.10.1
- Replace `golang/govulncheck-action` with `make security-check` in CI
- Add `make lint` step to CI
- Fix all lint issues across the codebase (formatting, `errors.New` vs `fmt.Errorf`, `any` vs `interface{}`, unused returns, stuttering names, Go 1.26 `new(expr)`, etc.)
- Remove unused `pkg/ptr` package (inlined by Go 1.26 `new(expr)`)
- Remove dead linter exclusions (staticcheck, gocritic, testifylint, dupl)

## Test plan
- [x] `make lint` passes
- [x] `go test ./...` passes
- [x] `make build` succeeds

Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/133
Reviewed-by: techknowlogick <techknowlogick@noreply.gitea.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

Makefile

@@ -3,6 +3,10 @@ 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
+GOVULNCHECK_PACKAGE ?= golang.org/x/vuln/cmd/govulncheck@v1
+GOFUMPT_PACKAGE ?= mvdan.cc/gofumpt@v0.9.2
+
 .PHONY: help
 help: ## Print this help message.
 	@echo "Usage: make [target]"
@@ -45,8 +49,29 @@ air: ## Install air for hot reload.
 dev: air ## run the application with hot reload
 	air --build.cmd "make build" --build.bin ./gitea-mcp
 
+.PHONY: lint
+lint: lint-go ## lint everything
+
+.PHONY: lint-fix
+lint-fix: lint-go-fix ## lint everything and fix issues
+
+.PHONY: lint-go
+lint-go: ## lint go files
+	$(GO) run $(GOLANGCI_LINT_PACKAGE) run
+
+.PHONY: lint-go-fix
+lint-go-fix: ## lint go files and fix issues
+	$(GO) run $(GOLANGCI_LINT_PACKAGE) run --fix
+
+.PHONY: security-check
+security-check: ## run security check
+	$(GO) run $(GOVULNCHECK_PACKAGE) -show color ./... || true
+
+.PHONY: tidy
+tidy: ## run go mod tidy
+	$(eval MIN_GO_VERSION := $(shell grep -Eo '^go\s+[0-9]+\.[0-9.]+' go.mod | cut -d' ' -f2))
+	$(GO) mod tidy -compat=$(MIN_GO_VERSION)
+
 .PHONY: vendor
-vendor: ## tidy and verify module dependencies
-	@echo 'Tidying and verifying module dependencies...'
-	go mod tidy
-	go mod verify
+vendor: tidy ## tidy and verify module dependencies
+	$(GO) mod verify