chore(lint): add golangci-lint v2 and fix all lint issues (#803)

## Summary
- Replace old `.golangci.yml` (v1 format) with v2 format, aligned with gitea's lint config
- Add `lint-go`, `lint-go-fix`, and `lint` Makefile targets using golangci-lint v2.10.1
- Replace `make vet` with `make lint` in CI workflow (lint includes vet)
- Fix all 35 lint issues: modernize (maps.Copy, range over int, any), perfsprint (errors.New), unparam (remove unused parameters), revive (var naming), staticcheck, forbidigo exclusion for cmd/
- Make `security-check` non-fatal (apply https://github.com/go-gitea/gitea/pull/36681)
- Remove dead gocritic exclusion rules (commentFormatting, exitAfterDefer)
- Remove dead linter exclusions and disabled checks (singleCaseSwitch, ST1003, QF1001, QF1006, QF1008, testifylint go-require/require-error, test file exclusions for dupl/errcheck/staticcheck/unparam)

## Test plan
- [x] `golangci-lint run` passes
- [x] `go build ./...` passes
- [x] `go test ./...` passes

---------

Co-authored-by: ChristopherHX <christopher.homberger@web.de>
Co-authored-by: Christopher Homberger <christopher.homberger@web.de>
Reviewed-on: https://gitea.com/gitea/act_runner/pulls/803
Reviewed-by: ChristopherHX <christopherhx@noreply.gitea.com>

internal/app/cmd/register.go

@@ -6,6 +6,7 @@ package cmd
 import (
 	"bufio"
 	"context"
+	"errors"
 	"fmt"
 	"os"
 	"os/signal"
@@ -107,10 +108,10 @@ type registerInputs struct {
 
 func (r *registerInputs) validate() error {
 	if r.InstanceAddr == "" {
-		return fmt.Errorf("instance address is empty")
+		return errors.New("instance address is empty")
 	}
 	if r.Token == "" {
-		return fmt.Errorf("token is empty")
+		return errors.New("token is empty")
 	}
 	if len(r.Labels) > 0 {
 		return validateLabels(r.Labels)