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>

operation/pull/pull_test.go

@@ -21,10 +21,10 @@ func TestEditPullRequestFn(t *testing.T) {
 	)
 
 	var (
-		mu          sync.Mutex
-		gotMethod   string
-		gotPath     string
-		gotBody     map[string]any
+		mu        sync.Mutex
+		gotMethod string
+		gotPath   string
+		gotBody   map[string]any
 	)
 
 	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
@@ -44,7 +44,7 @@ func TestEditPullRequestFn(t *testing.T) {
 			gotBody = body
 			mu.Unlock()
 			w.Header().Set("Content-Type", "application/json")
-			_, _ = w.Write([]byte(fmt.Sprintf(`{"number":%d,"title":"%s","state":"open"}`, index, body["title"])))
+			_, _ = w.Write(fmt.Appendf(nil, `{"number":%d,"title":"%s","state":"open"}`, index, body["title"]))
 		default:
 			http.NotFound(w, r)
 		}