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>

pkg/log/log.go

@@ -1,7 +1,6 @@
 package log
 
 import (
-	"fmt"
 	"os"
 	"sync"
 	"time"
@@ -35,14 +34,14 @@ func Default() *zap.Logger {
 			home = os.TempDir()
 		}
 
-		logDir := fmt.Sprintf("%s/.gitea-mcp", home)
+		logDir := home + "/.gitea-mcp"
 		if err := os.MkdirAll(logDir, 0o700); err != nil {
 			// Fallback to temp directory if creation fails
 			logDir = os.TempDir()
 		}
 
 		wss = append(wss, zapcore.AddSync(&lumberjack.Logger{
-			Filename:   fmt.Sprintf("%s/gitea-mcp.log", logDir),
+			Filename:   logDir + "/gitea-mcp.log",
 			MaxSize:    100,
 			MaxBackups: 10,
 			MaxAge:     30,